Polymer membranes for continuous analyte sensors

ABSTRACT

Devices and methods are described for providing continuous measurement of an analyte concentration. In some embodiments, the device has a sensing mechanism and a sensing membrane that includes at least one surface-active group-containing polymer and that is located over the sensing mechanism. The sensing membrane may have a bioprotective layer configured to substantially block the effect and/or influence of non-constant noise-causing species.

INCORPORATION BY REFERENCE TO RELATED APPLICATIONS

Any and all priority claims identified in the Application Data Sheet, orany correction thereto, are hereby incorporated by reference under 37CFR 1.57. This application is a continuation of U.S. application Ser.No. 14/058,154, filed Oct. 18, 2013, which is a continuation of U.S.application Ser. No. 12/718,299, filed Mar. 5, 2010, now U.S. Pat. No.8,583,204, which is a continuation-in-part of U.S. application Ser. No.12/413,231, filed Mar. 27, 2009, now abandoned, which claims the benefitof U.S. Provisional Application No. 61/040,594. Each of theaforementioned applications is incorporated by reference herein in itsentirety, and each is hereby expressly made a part of thisspecification.

BACKGROUND OF THE INVENTION

Electrochemical sensors are useful in chemistry and medicine todetermine the presence or concentration of a biological analyte. Suchsensors are useful, for example, to monitor glucose in diabetic patientsand lactate during critical care events. A variety of intravascular,transcutaneous and implantable sensors have been developed forcontinuously detecting and quantifying blood glucose values. Manyimplantable glucose sensors suffer from complications within the bodyand provide only short-term or less-than-accurate sensing of bloodglucose. Similarly, many transcutaneous and intravascular sensors haveproblems in accurately sensing and reporting back glucose valuescontinuously over extended periods of time, for example, due to noise onthe signal caused by interfering species or unknown noise-causingevents.

SUMMARY OF THE INVENTION

In a first aspect, a device for continuous measurement of an analyteconcentration is provided, the device comprising: a sensing mechanismconfigured to continuously measure a signal associated with an analyteconcentration in a host; and a membrane located over the sensingmechanism, wherein the membrane comprises a polyurethane and ahydrophilic portion; wherein the device is configured to provide, atanalyte concentrations of from about 40 mg/dL to about 400 mg/dL, alevel of accuracy corresponding to a mean absolute relative differenceof no more than about 8% over a sensor session of at least about 3 days,wherein one or more reference measurements associated with calculationof the mean absolute relative difference are determined by analysis ofblood.

In an embodiment of the first aspect, the sensor session is at leastabout 5 days.

In an embodiment of the first aspect, the sensor session is at leastabout 6 days.

In an embodiment of the first aspect, the sensor session is at leastabout 7 days.

In an embodiment of the first aspect, the sensor session is at leastabout 10 days.

In an embodiment of the first aspect, the mean absolute relativedifference is no more than about 7% over the sensor session.

In an embodiment of the first aspect, the membrane comprises an enzymeconfigured to react with the analyte.

In an embodiment of the first aspect, the membrane comprises a copolymercomprising a fluorocarbon segment.

In an embodiment of the first aspect, the membrane comprises a copolymercomprising a silicone segment.

In an embodiment of the first aspect, the membrane comprises apolycarbonate segment.

In a second aspect, a system for continuous measurement of an analyteconcentration is provided, the system comprising: a sensor comprising: asensing region configured to continuously produce sensor data associatedwith an analyte concentration in a host; and a membrane located over thesensing region, wherein the membrane comprises a polyurethane and ahydrophilic portion; a processor configured to process continuous sensordata; and a user interface configured to display information associatedwith continuous sensor data; wherein the sensor is configured toprovide, at analyte concentrations of from about 40 mg/dL to about 400mg/dL, a level of accuracy corresponding to a mean absolute relativedifference of no more than about 8% over a sensor session of at leastabout 3 days, wherein one or more reference measurements associated withcalculation of the mean absolute relative difference are determined byanalysis of blood.

In an embodiment of the second aspect, the sensor session is at leastabout 5 days.

In an embodiment of the second aspect, the sensor session is at leastabout 6 days.

In an embodiment of the second aspect, the sensor session is at leastabout 7 days.

In an embodiment of the second aspect, the sensor session is at leastabout 10 days.

In an embodiment of the second aspect, the mean absolute relativedifference is no more than about 7% over the sensor session.

In an embodiment of the second aspect, the membrane comprises an enzymeconfigured to react with the analyte.

In an embodiment of the second aspect, the membrane comprises acopolymer comprising a fluorocarbon segment.

In an embodiment of the second aspect, the membrane comprises acopolymer comprising a silicone segment.

In an embodiment of the second aspect, the membrane comprises apolycarbonate segment.

In a third aspect, a device for continuous measurement of an analyteconcentration is provided, the device comprising: a sensing mechanismconfigured to continuously measure a signal associated with an analyteconcentration in a host; and a membrane located over the sensingmechanism; wherein the device is configured to provide, at analyteconcentrations of from about 40 mg/dL to about 80 mg/dL, a level ofaccuracy of a mean absolute relative difference of no more than about10% over a sensor session of at least about 3 days, wherein one or morereference measurements associated with calculation of the mean absoluterelative difference are determined by analysis of blood; and wherein thedevice is configured to provide, at analyte concentrations of from about40 mg/dL to about 400 mg/dL, a level of accuracy of a mean absoluterelative difference of no more than about 10% over the sensor session,wherein one or more reference measurements associated with calculationof the mean absolute relative difference are determined by analysis ofblood.

In an embodiment of the third aspect, the membrane comprises an enzymeconfigured to react with the analyte.

In an embodiment of the third aspect, the membrane comprises apolyurethane and a hydrophilic portion.

In an embodiment of the third aspect, the membrane comprises a copolymercomprising a fluorocarbon segment.

In an embodiment of the third aspect, the membrane comprises a copolymercomprising a silicone segment.

In an embodiment of the third aspect, the membrane comprises a copolymercomprising a polycarbonate segment.

In a fourth aspect, a device for continuous measurement of an analyteconcentration is provided, the device comprising: a sensing mechanismconfigured to continuously measure a signal associated with an analyteconcentration in a host; and a membrane located over the sensingmechanism; wherein, over a sensor session of at least about 3 days, thedevice is configured to: provide a level of accuracy corresponding to afirst mean absolute relative difference value at analyte concentrationsof from about 40 mg/dL to about 80 mg/dL, wherein one or more referencemeasurements associated with calculation of the first mean absoluterelative difference are determined by analysis of blood; and provide alevel of accuracy corresponding to a second mean absolute relativedifference value at analyte concentrations of from about 40 mg/dL toabout 400 mg/dL, wherein one or more reference measurements associatedwith calculation of the second mean absolute relative difference aredetermined by analysis of blood; and wherein the first mean absoluterelative difference value is less than or about equal to the second meanabsolute relative difference value.

In an embodiment of the fourth aspect, the membrane comprises an enzymeconfigured to react with the analyte.

In an embodiment of the fourth aspect, the membrane comprises apolyurethane and a hydrophilic portion.

In an embodiment of the fourth aspect, the membrane comprises acopolymer comprising a fluorocarbon segment.

In an embodiment of the fourth aspect, the membrane comprises acopolymer comprising a silicone segment.

In an embodiment of the fourth aspect, the membrane comprises acopolymer comprising a polycarbonate segment.

In a fifth aspect, a system for continuous measurement of an analyteconcentration is provided, the system comprising: a sensor comprising asensing region configured to continuously produce sensor data associatedwith an analyte concentration in a host, wherein the sensor furthercomprises a membrane located over the sensing region; a processorconfigured to process continuous sensor data; and a user interfaceconfigured to display information associated with continuous sensordata; wherein the sensor is configured to provide, at analyteconcentrations of from about 40 mg/dL to about 80 mg/dL, a level ofaccuracy of a mean absolute relative difference of no more than about10% over a sensor session of at least about 3 days, wherein one or morereference measurements associated with calculation of the mean absoluterelative difference are determined by analysis of blood; and wherein thesensor is configured to provide, at analyte concentrations of from about40 mg/dL and about 400 mg/dL, a level of accuracy of a mean absoluterelative difference of no more than about 10% over the sensor session,wherein one or more reference measurements associated with calculationof the mean absolute relative difference are determined by analysis ofblood.

In an embodiment of the fifth aspect, the membrane comprises an enzymeconfigured to react with the analyte.

In an embodiment of the fifth aspect, the membrane comprises apolyurethane and a hydrophilic portion.

In an embodiment of the fifth aspect, the membrane comprises a copolymercomprising a fluorocarbon segment.

In an embodiment of the fifth aspect, the membrane comprises a copolymercomprising a silicone segment.

In an embodiment of the fifth aspect, the membrane comprises a copolymercomprising a polycarbonate segment.

In a sixth aspect, a system for continuous measurement of an analyteconcentration is provided, the system comprising: a sensor comprising asensing mechanism configured to continuously measure a signal associatedwith an analyte concentration in a host, wherein the sensor furthercomprises a membrane located over the sensing mechanism; a processorconfigured to process continuous sensor data; and a user interfaceconfigured to display information associated with continuous sensordata; wherein, over a sensor session of at least about 3 days, thesystem is configured to: provide a level of accuracy corresponding to afirst mean absolute relative difference value at analyte concentrationsof from about 40 mg/dL to about 80 mg/dL, wherein one or more referencemeasurements associated with calculation of the first mean absoluterelative difference are determined by analysis of blood; and provide alevel of accuracy corresponding to a second mean absolute relativedifference value at analyte concentrations of from about 40 mg/dL toabout 400 mg/dL, wherein one or more reference measurements associatedwith calculation of the second mean absolute relative difference aredetermined by analysis of blood; and wherein the first mean absoluterelative difference value is less than or about equal to the second meanabsolute relative difference value.

In an embodiment of the sixth aspect, the membrane comprises an enzymeconfigured to react with the analyte.

In an embodiment of the sixth aspect, the membrane comprises apolyurethane and a hydrophilic portion.

In an embodiment of the sixth aspect, the membrane comprises a copolymercomprising a fluorocarbon segment.

In an embodiment of the sixth aspect, the membrane comprises a copolymercomprising a silicone segment.

In an embodiment of the sixth aspect, the membrane comprises a copolymercomprising a polycarbonate segment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an expanded view of an exemplary embodiment of a continuousanalyte sensor.

FIGS. 2A-2C are cross-sectional views through the sensor of FIG. 1 online 2-2, illustrating various embodiments of the membrane system.

FIG. 3 is a graph illustrating the components of a signal measured by aglucose sensor (after sensor break-in was complete), in a non-diabeticvolunteer host.

FIG. 4A is a schematic view of a base polymer containing surface-activeend groups in one embodiment.

FIG. 4B is a schematic view of a bioprotective domain, showing aninterface in a biological environment (e.g., interstitial space orvascular space).

FIG. 5 is a graph illustrating in vivo test results comparing a controland test sensors bilaterally implanted in a human host, as described inExample 2.

FIGS. 6A and 6B are graphs illustrating in vivo test results fromcontrol (FIG. 6A) and test (FIG. 6B) sensors implanted bilaterally intoa rat, over a period of more than about 2 days.

FIG. 7 is a graph comparing the in vivo glucose sensitivity of a sensorimplanted in one rat with the in vitro glucose sensitivity of a sensorin glucose PBS solution, as described in Example 4.

FIG. 8 is a graph illustrating signals, following administration ofacetaminophen, received from an enzymatic electrode with a bioprotectivelayer formed with silicone-polycarbonate-urethane blended with PVP,compared to one formed with a conventional polyurethane membrane, asdescribed in Example 5.

FIGS. 9A and 9B are graphs illustrating the percentages of baselinesignal to total signal under various environments, as described inExample 6.

FIG. 10A is a graph illustrating the conversion function of a sensorwith a substantial background signal. FIG. 10B is a graph illustratingthe conversion function of a sensor similar to that associated with FIG.10A, but with a substantial reduction in the background signal. FIGS.10A and 10B both also display glucose signal amplitudes and baselinesignal amplitudes at certain glucose concentrations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description and examples describe in detail some exemplaryembodiments of devices and methods for providing continuous measurementof an analyte concentration. It should be appreciated that there arenumerous variations and modifications of the devices and methodsdescribed herein that are encompassed by the present invention.Accordingly, the description of a certain exemplary embodiment shouldnot be deemed to limit the scope of the present invention.

DEFINITIONS

In order to facilitate an understanding of the devices and methodsdescribed herein, a number of terms are defined below.

The term ‘analyte’ as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and is not to be limited to a special or customized meaning), andrefers without limitation to a substance or chemical constituent in abiological fluid (for example, blood, interstitial fluid, cerebralspinal fluid, lymph fluid, urine, sweat, saliva, etc.) that can beanalyzed. Analytes can include naturally occurring substances,artificial substances, metabolites, or reaction products. In someembodiments, the analyte for measurement by the sensing regions,devices, and methods is glucose. However, other analytes arecontemplated as well, including, but not limited to:acarboxyprothrombin; acylcarnitine; adenine phosphoribosyl transferase;adenosine deaminase; albumin; alpha-fetoprotein; amino acid profiles(arginine (Krebs cycle), histidine/urocanic acid, homocysteine,phenylalanine/tyrosine, tryptophan); andrenostenedione; antipyrine;arabinitol enantiomers; arginase; benzoylecgonine (cocaine);biotimidase; biopterin; c-reactive protein; carnitine; carnosinase; CD4;ceruloplasmin; chenodeoxycholic acid; chloroquine; cholesterol;cholinesterase; conjugated 1-β hydroxy-cholic acid; cortisol; creatinekinase; creatine kinase MM isoenzyme; cyclosporin A; d-penicillamine;de-ethylchloroquine; dehydroepiandrosterone sulfate; DNA (acetylatorpolymorphism, alcohol dehydrogenase, alpha 1-antitrypsin, cysticfibrosis, Duchenne/Becker muscular dystrophy, glucose-6-phosphatedehydrogenase, hemoglobin A, hemoglobin S, hemoglobin C, hemoglobin D,hemoglobin E, hemoglobin F, D-Punjab, beta-thalassemia, hepatitis Bvirus, HCMV, HIV-1, HTLV-1, Leber hereditary optic neuropathy, MCAD,RNA, PKU, Plasmodium vivax, sexual differentiation, 21-deoxycortisol);desbutylhalofantrine; dihydropteridine reductase; diptheria/tetanusantitoxin; erythrocyte arginase; erythrocyte protoporphyrin; esterase D;fatty acids/acylglycines; free β-human chorionic gonadotropin; freeerythrocyte porphyrin; free thyroxine (FT4); free tri-iodothyronine(FT3); fumarylacetoacetase; galactose/gal-1-phosphate;galactose-1-phosphate uridyltransferase; gentamicin; glucose-6-phosphatedehydrogenase; glutathione; glutathione perioxidase; glycocholic acid;glycosylated hemoglobin; halofantrine; hemoglobin variants;hexosaminidase A; human erythrocyte carbonic anhydrase I;17-alpha-hydroxyprogesterone; hypoxanthine phosphoribosyl transferase;immunoreactive trypsin; lactate; lead; lipoproteins ((a), B/A-1, β);lysozyme; mefloquine; netilmicin; phenobarbitone; phenyloin;phytanic/pristanic acid; progesterone; prolactin; prolidase; purinenucleoside phosphorylase; quinine; reverse tri-iodothyronine (rT3);selenium; serum pancreatic lipase; sissomicin; somatomedin C; specificantibodies (adenovirus, anti-nuclear antibody, anti-zeta antibody,arbovirus, Aujeszky's disease virus, dengue virus, Dracunculusmedinensis, Echinococcus granulosus, Entamoeba histolytica, enterovirus,Giardia duodenalisa, Helicobacter pylori, hepatitis B virus, herpesvirus, HIV-1, IgE (atopic disease), influenza virus, Leishmaniadonovani, leptospira, measles/mumps/rubella, Mycobacterium leprae,Mycoplasma pneumoniae, Myoglobin, Onchocerca volvulus, parainfluenzavirus, Plasmodium falciparum, poliovirus, Pseudomonas aeruginosa,respiratory syncytial virus, rickettsia (scrub typhus), Schistosomamansoni, Toxoplasma gondii, Trepenoma pallidium, Trypanosomacruzi/rangeli, vesicular stomatis virus, Wuchereria bancrofti, yellowfever virus); specific antigens (hepatitis B virus, HIV-1);succinylacetone; sulfadoxine; theophylline; thyrotropin (TSH); thyroxine(T4); thyroxine-binding globulin; trace elements; transferrin;UDP-galactose-4-epimerase; urea; uroporphyrinogen I synthase; vitamin A;white blood cells; and zinc protoporphyrin. Salts, sugar, protein, fat,vitamins, and hormones naturally occurring in blood or interstitialfluids can also constitute analytes in certain embodiments. The analytecan be naturally present in the biological fluid or endogenous, forexample, a metabolic product, a hormone, an antigen, an antibody, andthe like. Alternatively, the analyte can be introduced into the body orexogenous, for example, a contrast agent for imaging, a radioisotope, achemical agent, a fluorocarbon-based synthetic blood, or a drug orpharmaceutical composition, including but not limited to: insulin;ethanol; cannabis (marijuana, tetrahydrocannabinol, hashish); inhalants(nitrous oxide, amyl nitrite, butyl nitrite, chlorohydrocarbons,hydrocarbons); cocaine (crack cocaine); stimulants (amphetamines,methamphetamines, Ritalin, Cylert, Preludin, Didrex, PreState, Voranil,Sandrex, Plegine); depressants (barbituates, methaqualone, tranquilizerssuch as Valium, Librium, Miltown, Serax, Equanil, Tranxene);hallucinogens (phencyclidine, lysergic acid, mescaline, peyote,psilocybin); narcotics (heroin, codeine, morphine, opium, meperidine,Percocet, Percodan, Tussionex, Fentanyl, Darvon, Talwin, Lomotil);designer drugs (analogs of fentanyl, meperidine, amphetamines,methamphetamines, and phencyclidine, for example, Ecstasy); anabolicsteroids; and nicotine. The metabolic products of drugs andpharmaceutical compositions are also contemplated analytes. Analytessuch as neurochemicals and other chemicals generated within the body canalso be analyzed, such as, for example, ascorbic acid, uric acid,dopamine, noradrenaline, 3-methoxytyramine (3MT),3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA),5-hydroxytryptamine (5HT), and 5-hydroxyindoleacetic acid (FHIAA).

The phrase ‘continuous (or continual) analyte sensing’ as used herein isa broad term, and is to be given its ordinary and customary meaning to aperson of ordinary skill in the art (and is not to be limited to aspecial or customized meaning), and refers without limitation to theperiod in which monitoring of analyte concentration is continuously,continually, and or intermittently (but regularly) performed, forexample, about every 5 to 10 minutes.

The terms ‘operable connection,’ ‘operably connected,’ and ‘operablylinked’ as used herein are broad terms, and are to be given theirordinary and customary meaning to a person of ordinary skill in the art(and are not to be limited to a special or customized meaning), andrefer without limitation to one or more components linked to anothercomponent(s) in a manner that allows transmission of signals between thecomponents. For example, one or more electrodes can be used to detectthe amount of analyte in a sample and convert that information into asignal; the signal can then be transmitted to a circuit. In this case,the electrode is ‘operably linked’ to the electronic circuitry.

The term ‘host’ as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to animals (e.g., humans) and plants.

The terms ‘electrochemically reactive surface’ and ‘electroactivesurface’ as used herein are broad terms, and are to be given theirordinary and customary meaning to a person of ordinary skill in the art(and are not to be limited to a special or customized meaning), andrefer without limitation to the surface of an electrode where anelectrochemical reaction takes place. As one example, in a workingelectrode, H₂O₂ (hydrogen peroxide) produced by an enzyme-catalyzedreaction of an analyte being detected reacts and thereby creates ameasurable electric current. For example, in the detection of glucose,glucose oxidase produces H₂O₂ as a byproduct. The H₂O₂ reacts with thesurface of the working electrode to produce two protons (2H⁺), twoelectrons (2e⁻), and one molecule of oxygen (O₂), which produces theelectric current being detected. In the case of the counter electrode, areducible species, for example, O₂ is reduced at the electrode surfacein order to balance the current being generated by the workingelectrode.

The terms ‘sensing region,’ ‘sensor’, and ‘sensing mechanism’ as usedherein are broad terms, and are to be given their ordinary and customarymeaning to a person of ordinary skill in the art (and are not to belimited to a special or customized meaning), and refer withoutlimitation to the region or mechanism of a monitoring device responsiblefor the detection of a particular analyte.

The terms ‘raw data stream’ and ‘data stream’ as used herein are broadterms, and are to be given their ordinary and customary meaning to aperson of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation to ananalog or digital signal directly related to the measured glucoseconcentration from the glucose sensor. In one example, the raw datastream is digital data in ‘counts’ converted by an A/D converter from ananalog signal (for example, voltage or amps) representative of a glucoseconcentration. The terms broadly encompass a plurality of time spaceddata points from a substantially continuous glucose sensor, whichcomprises individual measurements taken at time intervals ranging fromfractions of a second up to, for example, 1, 2, or 5 minutes or longer.

The term ‘counts’ as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to a unit of measurement of a digital signal.In one example, a raw data stream measured in counts is directly relatedto a voltage (for example, converted by an A/D converter), which isdirectly related to current from the working electrode. In anotherexample, counter electrode voltage measured in counts is directlyrelated to a voltage.

The term ‘electrical potential’ as used herein is a broad term, and isto be given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to the electrical potentialdifference between two points in a circuit which is the cause of theflow of a current.

The phrase ‘distal to’ as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to the spatial relationshipbetween various elements in comparison to a particular point ofreference. For example, some embodiments of a sensor include a membranesystem having a bioprotective domain and an enzyme domain. If the sensoris deemed to be the point of reference and the bioprotective domain ispositioned farther from the sensor than the enzyme domain, then thebioprotective domain is more distal to the sensor than the enzymedomain.

The phrase ‘proximal to’ as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to the spatial relationshipbetween various elements in comparison to a particular point ofreference. For example, some embodiments of a device include a membranesystem having a bioprotective domain and an enzyme domain. If the sensoris deemed to be the point of reference and the enzyme domain ispositioned nearer to the sensor than the bioprotective domain, then theenzyme domain is more proximal to the sensor than the bioprotectivedomain.

The terms ‘interferents’ and ‘interfering species’ as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation to effectsor species that interfere with the measurement of an analyte of interestin a sensor to produce a signal that does not accurately represent theanalyte measurement. In an exemplary electrochemical sensor, interferingspecies can include compounds with an oxidation potential that overlapswith that of the analyte to be measured.

The term ‘domain’ as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to regions of a membrane that can be layers,uniform or non-uniform gradients (i.e., anisotropic) or provided asportions of the membrane.

The terms ‘sensing membrane’ and ‘membrane system’ as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refers without limitation to apermeable or semi-permeable membrane that can comprise one or moredomains and constructed of materials of a few microns thickness or more,which are permeable to oxygen and may or may not be permeable to ananalyte of interest. In one example, the sensing membrane or membranesystem may comprise an immobilized glucose oxidase enzyme, which enablesan electrochemical reaction to occur to measure a concentration ofglucose.

The term ‘baseline’ as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and is not to be limited to a special or customized meaning), andrefers without limitation to the component of an analyte sensor signalthat is not related to the analyte concentration. In one example of aglucose sensor, the baseline is composed substantially of signalcontribution due to factors other than glucose (for example, interferingspecies, non-reaction-related hydrogen peroxide, or other electroactivespecies with an oxidation potential that overlaps with hydrogenperoxide). In some embodiments wherein a calibration is defined bysolving for the equation y=mx+b, the value of b represents the baselineof the signal.

The term ‘sensitivity’ as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to an amount of electricalcurrent produced by a predetermined amount (unit) of the measuredanalyte. For example, in one embodiment, a sensor has a sensitivity (orslope) of from about 1 to about 100 picoAmps of current for every 1mg/dL of glucose analyte.

The term ‘sensor session’ is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to the period of time the sensor is applied to(e.g., implanted in) the host or is being used to obtain sensor values.For example, in some embodiments, a sensor session extends from the timeof sensor implantation (e.g., including insertion of the sensor intosubcutaneous tissue and placing the sensor into fluid communication witha host's circulatory system) to the time when the sensor is removed.

As employed herein, the following abbreviations apply: Eq and Eqs(equivalents); mEq (milliequivalents); M (molar); mM (millimolar) μM(micromolar); N (Normal); mol (moles); mmol (millimoles); μmol(micromoles); nmol (nanomoles); g (grams); mg (milligrams); μg(micrograms); Kg (kilograms); L (liters); mL (milliliters); dL(deciliters); μL (microliters); cm (centimeters); mm (millimeters); μm(micrometers); nm (nanometers); h and hr (hours); min. (minutes); s andsec. (seconds); ° C. (degrees Centigrade).

Overview

Membrane systems of the preferred embodiments are suitable for use withimplantable devices in contact with a biological fluid. For example, themembrane systems can be utilized with implantable devices, such asdevices for monitoring and determining analyte levels in a biologicalfluid, for example, devices for monitoring glucose levels forindividuals having diabetes. In some embodiments, the analyte-measuringdevice is a continuous device. The analyte-measuring device can employany suitable sensing element to provide the raw signal, including butnot limited to those involving enzymatic, chemical, physical,electrochemical, spectrophotometric, polarimetric, calorimetric,radiometric, immunochemical, or like elements.

Although some of the description that follows is directed atglucose-measuring devices, including the described membrane systems andmethods for their use, these membrane systems are not limited to use indevices that measure or monitor glucose. These membrane systems aresuitable for use in any of a variety of devices, including, for example,devices that detect and quantify other analytes present in biologicalfluids (e.g., cholesterol, amino acids, alcohol, galactose, andlactate), cell transplantation devices (see, for example, U.S. Pat. No.6,015,572, U.S. Pat. No. 5,964,745, and U.S. Pat. No. 6,083,523), drugdelivery devices (see, for example, U.S. Pat. No. 5,458,631, U.S. Pat.No. 5,820,589, and U.S. Pat. No. 5,972,369), and the like.

In one embodiment, the analyte sensor is an implantable glucose sensor,such as described with reference to U.S. Pat. No. 6,001,067 and U.S.Patent Publication No. US-2005-0027463-A1, each of which is incorporatedherein by reference in its entirety. In another embodiment, the analytesensor is a glucose sensor, such as described with reference to U.S.Patent Publication No. US-2006-0020187-A1, which is incorporated hereinby reference in its entirety. In still other embodiments, the sensor isconfigured to be implanted in a host vessel or extra-corporeally, suchas is described in U.S. Patent Publication No. US-2007-0027385-A1, U.S.Patent Publication No. US-2008-0119703-A1, U.S. Patent Publication No.US-20080108942-A1, and U.S. Patent Publication No. US-2007-0197890-A1,all of which are incorporated herein by reference in their entirety. Insome embodiments, the sensor is configured as a dual-electrode sensor,such as described in U.S. Patent Publication No. US-2005-0143635-A1,U.S. Patent Publication No. US-2007-0027385-A1, U.S. Patent PublicationNo. US-2007-0213611-A1, and U.S. Patent Publication No.US-2008-0083617-A1, which are incorporated herein by reference in theirentirety. In one alternative embodiment, the continuous glucose sensorcomprises a sensor such as described in U.S. Pat. No. 6,565,509 to Sayet al., for example. In another alternative embodiment, the continuousglucose sensor comprises a subcutaneous sensor such as described withreference to U.S. Pat. No. 6,579,690 to Bonnecaze et al. or U.S. Pat.No. 6,484,046 to Say et al., for example. In another alternativeembodiment, the continuous glucose sensor comprises a refillablesubcutaneous sensor such as described with reference to U.S. Pat. No.6,512,939 to Colvin et al., for example. In yet another alternativeembodiment, the continuous glucose sensor comprises an intravascularsensor such as described with reference to U.S. Pat. No. 6,477,395 toSchulman et al., for example. In another alternative embodiment, thecontinuous glucose sensor comprises an intravascular sensor such asdescribed with reference to U.S. Pat. No. 6,424,847 to Mastrototaro etal. In some embodiments, the electrode system can be used with any of avariety of known in vivo analyte sensors or monitors, such as U.S. Pat.No. 7,157,528 to Ward; U.S. Pat. No. 6,212,416 to Ward et al.; U.S. Pat.No. 6,119,028 to Schulman et al.; U.S. Pat. No. 6,400,974 to Lesho; U.S.Pat. No. 6,595,919 to Berner et al.; U.S. Pat. No. 6,141,573 to Kurniket al.; U.S. Pat. No. 6,122,536 to Sun et al.; European PatentApplication EP 1153571 to Varall et al.; U.S. Pat. No. 6,512,939 toColvin et al.; U.S. Pat. No. 5,605,152 to Slate et al.; U.S. Pat. No.4,431,004 to Bessman et al.; U.S. Pat. No. 4,703,756 to Gough et al.;U.S. Pat. No. 6,514,718 to Heller et al.; U.S. Pat. No. 5,985,129 toGough et al.; WO Patent Application Publication No. 04/021877 to Caduff;U.S. Pat. No. 5,494,562 to Maley et al.; U.S. Pat. No. 6,120,676 toHeller et al.; and U.S. Pat. No. 6,542,765 to Guy et al. In general, itis understood that the disclosed embodiments are applicable to a varietyof continuous analyte measuring device configurations.

In some embodiments, a long term sensor (e.g., wholly implantable orintravascular) is configured to function for a time period of from about30 days or less to about one year or more (e.g., a sensor session). Insome embodiments, a short term sensor (e.g., one that is transcutaneousor intravascular) is configured and arranged to function for a timeperiod of from about a few hours to about 30 days, including a timeperiod of about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28 or 29 days (e.g., asensor session). As used herein, the term ‘sensor session’ is a broadterm and refers without limitation to the period of time the sensor isapplied to (e.g., implanted in) the host or is being used to obtainsensor values. For example, in some embodiments, a sensor sessionextends from the time of sensor implantation (e.g., including insertionof the sensor into subcutaneous tissue and placing the sensor into fluidcommunication with a host's circulatory system) to the time when thesensor is removed.

Exemplary Glucose Sensor Configuration

FIG. 1 is an expanded view of an exemplary embodiment of a continuousanalyte sensor 34, also referred to as an analyte sensor, illustratingthe sensing mechanism. In some embodiments, the sensing mechanism isadapted for insertion under the host's skin, and the remaining body ofthe sensor (e.g., electronics, etc.) can reside ex vivo. In theillustrated embodiment, the analyte sensor 34 includes two electrodes,i.e., a working electrode 38 and at least one additional electrode 30,which may function as a counter or reference electrode, hereinafterreferred to as the reference electrode 30.

It is contemplated that the electrode may be formed to have any of avariety of cross-sectional shapes. For example, in some embodiments, theelectrode may be formed to have a circular or substantially circularshape, but in other embodiments, the electrode may be formed to have across-sectional shape that resembles an ellipse, a polygon (e.g.,triangle, square, rectangle, parallelogram, trapezoid, pentagon,hexagon, octagon), or the like. In various embodiments, thecross-sectional shape of the electrode may be symmetrical, but in otherembodiments, the cross-sectional shape may be asymmetrical. In someembodiments, each electrode may be formed from a fine wire with adiameter of from about 0.001 or less to about 0.050 inches or more, forexample, and is formed from, e.g., a plated insulator, a plated wire, orbulk electrically conductive material. In some embodiments, the wireused to form a working electrode may be about 0.002, 0.003, 0.004,0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.015, 0.02, 0.025, 0.03,0.035, 0.04 or 0.045 inches in diameter. In some embodiments, theworking electrode may comprise a wire formed from a conductive material,such as platinum, platinum-black, platinum-iridium, palladium, graphite,gold, carbon, conductive polymer, alloys, or the like. Although theillustrated electrode configuration and associated text describe onemethod of forming a sensor, any of a variety of known sensorconfigurations can be employed with the analyte sensor system.

The working electrode 38 is configured to measure the concentration ofan analyte, such as, but not limited to glucose, uric acid, cholesterol,lactate, and the like. In an enzymatic electrochemical sensor fordetecting glucose, for example, the working electrode may measure thehydrogen peroxide produced by an enzyme catalyzed reaction of theanalyte being detected and creates a measurable electric current. Forexample, in the detection of glucose wherein glucose oxidase (GOX)produces H₂O₂ as a byproduct, the H₂O₂ reacts with the surface of theworking electrode producing two protons (2H⁺), two electrons (2e⁻) andone molecule of oxygen (O₂), which produces the electric current beingdetected.

An insulator may be provided to electrically insulate the working andreference electrodes. In this exemplary embodiment, the workingelectrode 38 is covered with an insulating material, for example, anon-conductive polymer. Dip-coating, spray-coating, vapor-deposition, orother coating or deposition techniques can be used to deposit theinsulating material on the working electrode. In one embodiment, theinsulating material comprises parylene, which can be an advantageouspolymer coating because of its strength, lubricity, and electricalinsulation properties. Generally, parylene is produced by vapordeposition and polymerization of para-xylylene (or its substitutedderivatives). However, any suitable insulating material can be used, forexample, fluorinated polymers, polyethyleneterephthalate, polyurethane,polyimide, other nonconducting polymers, or the like. Glass or ceramicmaterials can also be employed. Other materials suitable for use includesurface energy modified coating systems such as those marketed under thetrade names AMC18, AMC148, AMC141, and AMC321 by Advanced MaterialsComponents Express of Bellafonte, Pa. In some alternative embodiments,however, the working electrode may not require a coating of insulator.

In some embodiments, the reference electrode 30, which may function as areference electrode alone, or as a dual reference and counter electrode,is formed from silver, silver/silver chloride, or the like. In someembodiments, the electrodes are juxtapositioned or twisted with oraround each other, but it is contemplated, however, that otherconfigurations are also possible. In one embodiment, the referenceelectrode 30 is helically wound around the working electrode 38. Theassembly of wires may then be optionally coated together with aninsulating material, similar to that described above, in order toprovide an insulating attachment (e.g., securing together of the workingand reference electrodes).

In embodiments wherein an outer insulator is disposed, a portion of thecoated assembly structure can be stripped or otherwise removed, forexample, by hand, excimer lasing, chemical etching, laser ablation,grit-blasting, or the like, to expose the electroactive surfaces.Alternatively, a portion of the electrode can be masked prior todepositing the insulator in order to maintain an exposed electroactivesurface area.

In some embodiments, a radial window is formed through the insulatingmaterial to expose a circumferential electroactive surface of theworking electrode. Additionally, sections of electroactive surface ofthe reference electrode are exposed. For example, the sections ofelectroactive surface can be masked during deposition of an outerinsulating layer or etched after deposition of an outer insulatinglayer. In some applications, cellular attack or migration of cells tothe sensor can cause reduced sensitivity or function of the device,particularly after the first day of implantation. However, when theexposed electroactive surface is distributed circumferentially about thesensor (e.g., as in a radial window), the available surface area forreaction can be sufficiently distributed so as to minimize the effect oflocal cellular invasion of the sensor on the sensor signal.Alternatively, a tangential exposed electroactive window can be formed,for example, by stripping only one side of the coated assemblystructure. In other alternative embodiments, the window can be providedat the tip of the coated assembly structure such that the electroactivesurfaces are exposed at the tip of the sensor. Other methods andconfigurations for exposing electroactive surfaces can also be employed.

In some alternative embodiments, additional electrodes can be includedwithin the assembly, for example, a three-electrode system (working,reference, and counter electrodes) and an additional working electrode(e.g., an electrode which can be used to generate oxygen, which isconfigured as a baseline subtracting electrode, or which is configuredfor measuring additional analytes). U.S. Pat. No. 7,081,195, U.S. PatentPublication No. US-2005-0143635-A1 and U.S. Patent Publication No.US-2007-0027385-A1, each of which are incorporated herein by reference,describe some systems and methods for implementing and using additionalworking, counter, and reference electrodes. In one implementationwherein the sensor comprises two working electrodes, the two workingelectrodes are juxtapositioned, around which the reference electrode isdisposed (e.g., helically wound). In some embodiments wherein two ormore working electrodes are provided, the working electrodes can beformed in a double-, triple-, quad-, etc. helix configuration along thelength of the sensor (for example, surrounding a reference electrode,insulated rod, or other support structure). The resulting electrodesystem can be configured with an appropriate membrane system, whereinthe first working electrode is configured to measure a first signalcomprising glucose and baseline signals, and the additional workingelectrode is configured to measure a baseline signal consisting of thebaseline signal only. In these embodiments, the second working electrodemay be configured to be substantially similar to the first workingelectrode, but without an enzyme disposed thereon. In this way, thebaseline signal can be determined and subtracted from the first signalto generate a difference signal, i.e., a glucose-only signal that issubstantially not subject to fluctuations in the baseline or interferingspecies on the signal, such as described in U.S. Patent Publication No.US-2005-0143635-A1, U.S. Patent Publication No. US-2007-0027385-A1, andU.S. Patent Publication No. US-2007-0213611-A1, and U.S. PatentPublication No. US-2008-0083617-A1, which are incorporated herein byreference in their entirety.

It has been found that in some electrode systems involving two workingelectrodes, i.e., in some dual-electrode systems, the working electrodesmay be slightly different from each other. For instance, two workingelectrodes, even when manufactured from a single facility may slightlydiffer in thickness or permeability because of the electrodes' highsensitivity to environmental conditions (e.g., temperature, humidity)during fabrication. Accordingly, the working electrodes of adual-electrode system may have varying diffusion, membrane thickness,and diffusion characteristics. As a result, the above-describeddifference signal (i.e., a glucose-only signal, generated fromsubtracting the baseline signal from the first signal) may not becompletely accurate. To mitigate this, it is contemplated that in somedual-electrode systems, both working electrodes may be fabricated withone or more membranes that each includes a bioprotective layer, which isdescribed in more detail elsewhere herein. Example 6 below describes indetail the results of reduction of interference-related signals achievedwith one embodiment in which the sensor comprises two workingelectrodes, each of which is covered by a bioprotective layer.

It is contemplated that the sensing region may include any of a varietyof electrode configurations. For example, in some embodiments, inaddition to one or more glucose-measuring working electrodes, thesensing region may also include a reference electrode or otherelectrodes associated with the working electrode. In these particularembodiments, the sensing region may also include a separate reference orcounter electrode associated with one or more optional auxiliary workingelectrodes. In other embodiments, the sensing region may include aglucose-measuring working electrode, an auxiliary working electrode, twocounter electrodes (one for each working electrode), and one sharedreference electrode. In yet other embodiments, the sensing region mayinclude a glucose-measuring working electrode, an auxiliary workingelectrode, two reference electrodes, and one shared counter electrode.

U.S. Patent Publication No. US-2008-0119703-A1 and U.S. PatentPublication No. US-2005-0245799-A1 describe additional configurationsfor using the continuous sensor in different body locations. In someembodiments, the sensor is configured for transcutaneous implantation inthe host. In alternative embodiments, the sensor is configured forinsertion into the circulatory system, such as a peripheral vein orartery. However, in other embodiments, the sensor is configured forinsertion into the central circulatory system, such as but not limitedto the vena cava. In still other embodiments, the sensor can be placedin an extracorporeal circulation system, such as but not limited to anintravascular access device providing extracorporeal access to a bloodvessel, an intravenous fluid infusion system, an extracorporeal bloodchemistry analysis device, a dialysis machine, a heart-lung machine(i.e., a device used to provide blood circulation and oxygenation whilethe heart is stopped during heart surgery), etc. In still otherembodiments, the sensor can be configured to be wholly implantable, asdescribed in U.S. Pat. No. 6,001,067.

FIG. 2A is a cross-sectional view through the sensor of FIG. 1 on line2-2, illustrating one embodiment of the membrane system 32. In thisparticular embodiment, the membrane system includes an enzyme domain 42,a diffusion resistance domain 44, and a bioprotective domain 46 locatedaround the working electrode 38, all of which are described in moredetail elsewhere herein. In some embodiments, a unitary diffusionresistance domain and bioprotective domain may be included in themembrane system (e.g., wherein the functionality of both domains isincorporated into one domain, i.e., the bioprotective domain). In someembodiments, the sensor is configured for short-term implantation (e.g.,from about 1 to 30 days). However, it is understood that the membranesystem 32 can be modified for use in other devices, for example, byincluding only one or more of the domains, or additional domains.

In some embodiments, the membrane system may include a bioprotectivedomain 46, also referred to as a cell-impermeable domain or biointerfacedomain, comprising a surface-modified base polymer as described in moredetail elsewhere herein. However, the sensing membranes 32 of someembodiments can also include a plurality of domains or layers including,for example, an electrode domain (e.g., as illustrated in the FIG. 2C),an interference domain (e.g., as illustrated in FIG. 2B), or a celldisruptive domain (not shown), such as described in more detailelsewhere herein and in U.S. Patent Publication No. US-2006-0036145-A1,which is incorporated herein by reference in its entirety.

It is to be understood that sensing membranes modified for othersensors, for example, may include fewer or additional layers. Forexample, in some embodiments, the membrane system may comprise oneelectrode layer, one enzyme layer, and two bioprotective layers, but inother embodiments, the membrane system may comprise one electrode layer,two enzyme layers, and one bioprotective layer. In some embodiments, thebioprotective layer may be configured to function as the diffusionresistance domain and control the flux of the analyte (e.g., glucose) tothe underlying membrane layers.

In some embodiments, one or more domains of the sensing membranes may beformed from materials such as silicone, polytetrafluoroethylene,polyethylene-co-tetrafluoroethylene, polyolefin, polyester,polycarbonate, biostable polytetrafluoroethylene, homopolymers,copolymers, terpolymers of polyurethanes, polypropylene (PP),polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), polybutyleneterephthalate (PBT), polymethylmethacrylate (PMMA), polyether etherketone (PEEK), polyurethanes, cellulosic polymers, poly(ethylene oxide),poly(propylene oxide) and copolymers and blends thereof, polysulfonesand block copolymers thereof including, for example, di-block,tri-block, alternating, random and graft copolymers.

In some embodiments, the sensing membrane can be deposited on theelectroactive surfaces of the electrode material using known thin orthick film techniques (for example, spraying, electro-depositing,dipping, or the like). The sensing membrane located over the workingelectrode does not have to have the same structure as the sensingmembrane located over the reference electrode; for example, the enzymedomain deposited over the working electrode does not necessarily need tobe deposited over the reference or counter electrodes.

Although the exemplary embodiments illustrated in FIGS. 2A-2C involvecircumferentially extending membrane systems, the membranes describedherein may be applied to any planar or non-planar surface, for example,the substrate-based sensor structure of U.S. Pat. No. 6,565,509 to Sayet al.

Sensor Electronics

In general, analyte sensor systems have electronics associatedtherewith, also referred to as a ‘computer system’ that can includehardware, firmware, or software that enable measurement and processingof data associated with analyte levels in the host. In one exemplaryembodiment of an electrochemical sensor, the electronics include apotentiostat, a power source for providing power to the sensor, andother components useful for signal processing. In additionalembodiments, some or all of the electronics can be in wired or wirelesscommunication with the sensor or other portions of the electronics. Forexample, a potentiostat disposed on the device can be wired to theremaining electronics (e.g., a processor, a recorder, a transmitter, areceiver, etc.), which reside on the bedside. In another example, someportion of the electronics is wirelessly connected to another portion ofthe electronics (e.g., a receiver), such as by infrared (IR) or RF. Itis contemplated that other embodiments of electronics may be useful forproviding sensor data output, such as those described in U.S. PatentPublication No. US-2005-0192557-A1, U.S. Patent Publication No.US-2005-0245795-A1; U.S. Patent Publication No. US-2005-0245795-A1, andU.S. Patent Publication No. US-2005-0245795-A1, U.S. Patent PublicationNo. US-2008-0119703-A1, and U.S. Patent Publication No.US-2008-0108942-A1, each of which is incorporated herein by reference inits entirety.

In one preferred embodiment, a potentiostat is operably connected to theelectrode(s) (such as described elsewhere herein), which biases thesensor to enable measurement of a current signal indicative of theanalyte concentration in the host (also referred to as the analogportion). In some embodiments, the potentiostat includes a resistor thattranslates the current into voltage. In some alternative embodiments, acurrent to frequency converter is provided that is configured tocontinuously integrate the measured current, for example, using a chargecounting device. In some embodiments, the electronics include an A/Dconverter that digitizes the analog signal into a digital signal, alsoreferred to as ‘counts’ for processing. Accordingly, the resulting rawdata stream in counts, also referred to as raw sensor data, is directlyrelated to the current measured by the potentiostat.

In general, the electronics include a processor module that includes thecentral control unit that controls the processing of the sensor system.In some embodiments, the processor module includes a microprocessor,however a computer system other than a microprocessor can be used toprocess data as described herein, for example an ASIC can be used forsome or all of the sensor's central processing. The processor typicallyprovides semi-permanent storage of data, for example, storing data suchas sensor identifier (ID) and programming to process data streams (forexample, programming for data smoothing or replacement of signalartifacts such as is described in U.S. Patent Publication No.US-2005-0043598-A1). The processor additionally can be used for thesystem's cache memory, for example for temporarily storing recent sensordata. In some embodiments, the processor module comprises memory storagecomponents such as ROM, RAM, dynamic-RAM, static-RAM, non-static RAM,EEPROM, rewritable ROMs, flash memory, and the like.

In some embodiments, the processor module comprises a digital filter,for example, an infinite impulse response (IIR) or finite impulseresponse (FIR) filter, configured to smooth the raw data stream.Generally, digital filters are programmed to filter data sampled at apredetermined time interval (also referred to as a sample rate). In someembodiments, wherein the potentiostat is configured to measure theanalyte at discrete time intervals, these time intervals determine thesample rate of the digital filter. In some alternative embodiments,wherein the potentiostat is configured to continuously measure theanalyte, for example, using a current-to-frequency converter asdescribed above, the processor module can be programmed to request adigital value from the A/D converter at a predetermined time interval,also referred to as the acquisition time. In these alternativeembodiments, the values obtained by the processor are advantageouslyaveraged over the acquisition time due the continuity of the currentmeasurement. Accordingly, the acquisition time determines the samplerate of the digital filter.

In some embodiments, the processor module is configured to build thedata packet for transmission to an outside source, for example, an RFtransmission to a receiver. Generally, the data packet comprises aplurality of bits that can include a preamble, a unique identifieridentifying the electronics unit, the receiver, or both, (e.g., sensorID code), data (e.g., raw data, filtered data, or an integrated value)or error detection or correction. Preferably, the data (transmission)packet has a length of from about 8 bits to about 128 bits, preferablyabout 48 bits; however, larger or smaller packets can be desirable incertain embodiments. The processor module can be configured to transmitany combination of raw or filtered data. In one exemplary embodiment,the transmission packet contains a fixed preamble, a unique ID of theelectronics unit, a single five-minute average (e.g., integrated) sensordata value, and a cyclic redundancy code (CRC).

In some embodiments, the processor further performs the processing, suchas storing data, analyzing data streams, calibrating analyte sensordata, estimating analyte values, comparing estimated analyte values withtime corresponding measured analyte values, analyzing a variation ofestimated analyte values, downloading data, and controlling the userinterface by providing analyte values, prompts, messages, warnings,alarms, and the like. In such cases, the processor includes hardware andsoftware that performs the processing described herein, for exampleflash memory provides permanent or semi-permanent storage of data,storing data such as sensor ID, receiver ID, and programming to processdata streams (for example, programming for performing estimation andother algorithms described elsewhere herein) and random access memory(RAM) stores the system's cache memory and is helpful in dataprocessing. Alternatively, some portion of the data processing (such asdescribed with reference to the processor elsewhere herein) can beaccomplished at another (e.g., remote) processor and can be configuredto be in wired or wireless connection therewith.

In some embodiments, an output module, which is integral with oroperatively connected with the processor, includes programming forgenerating output based on the data stream received from the sensorsystem and it's processing incurred in the processor. In someembodiments, output is generated via a user interface.

Noise

Generally, implantable sensors measure a signal related to an analyte ofinterest in a host. For example, an electrochemical sensor can measureglucose, creatinine, or urea in a host, such as an animal (e.g., ahuman). Generally, the signal is converted mathematically to a numericvalue indicative of analyte status, such as analyte concentration, asdescribed in more detail elsewhere herein. In general, the signalgenerated by conventional analyte sensors contains some noise. Noise isclinically important because it can induce error and can reduce sensorperformance, such as by providing a signal that causes the analyteconcentration to appear higher or lower than the actual analyteconcentration. For example, upward or high noise (e.g., noise thatcauses the signal to increase) can cause the reading of the host'sglucose concentration to appear higher than the actual value, which inturn can lead to improper treatment decisions. Similarly, downward orlow noise (e.g., noise that causes the signal to decrease) can cause thereading of the host's glucose concentration to appear lower than itsactual value, which in turn can also lead to improper treatmentdecisions. Accordingly, noise reduction is desirable.

In general, the signal detected by the sensor can be broken down intoits component parts. For example, in an enzymatic electrochemicalanalyte sensor, preferably after sensor break-in is complete, the totalsignal can be divided into an ‘analyte component,’ which isrepresentative of analyte (e.g., glucose) concentration, and a ‘noisecomponent,’ which is caused by non-analyte-related species that have aredox potential that substantially overlaps with the redox potential ofthe analyte (or measured species, e.g., H₂O₂) at an applied voltage. Thenoise component can be further divided into its component parts, e.g.,constant and non-constant noise. It is not unusual for a sensor toexperience a certain level of noise. In general, ‘constant noise’ (alsoreferred to as constant background or baseline) is caused bynon-analyte-related factors that are relatively stable over time,including but not limited to electroactive species that arise fromgenerally constant (e.g., daily) metabolic processes. Constant noise canvary widely between hosts. In contrast, ‘non-constant noise’ (alsoreferred to as non-constant background) is generally caused bynon-constant, non-analyte-related species (e.g., non-constantnoise-causing electroactive species) that may arise during transientevents, such as during host metabolic processes (e.g., wound healing orin response to an illness), or due to ingestion of certain compounds(e.g., certain drugs). In some circumstances, noise can be caused by avariety of noise-causing electroactive species, which are discussed indetail elsewhere herein.

FIG. 3 is a graph illustrating the components of a signal measured by atranscutaneous glucose sensor (after sensor break-in was complete), in anon-diabetic volunteer host. The Y-axis indicates the signal amplitude(in counts) detected by the sensor. The total signal collected by thesensor is represented by line 1000, which includes components related toglucose, constant noise, and non-constant noise, which are described inmore detail elsewhere herein. In some embodiments, the total signal is araw data stream, which can include an averaged or integrated signal, forexample, using a charge-counting device.

The non-constant noise component of the total signal is represented byline 1010. The non-constant noise component 1010 of the total signal1000 can be obtained by filtering the total signal 1000 to obtain afiltered signal 1020 using any of a variety of known filteringtechniques, and then subtracting the filtered signal 1020 from the totalsignal 1000. In some embodiments, the total signal can be filtered usinglinear regression analysis of the n (e.g., 10) most recent sampledsensor values. In some embodiments, the total signal can be filteredusing non-linear regression. In some embodiments, the total signal canbe filtered using a trimmed regression, which is a linear regression ofa trimmed mean (e.g., after rejecting wide excursions of any point fromthe regression line). In this embodiment, after the sensor recordsglucose measurements at a predetermined sampling rate (e.g., every 30seconds), the sensor calculates a trimmed mean (e.g., removes highestand lowest measurements from a data set) and then regresses theremaining measurements to estimate the glucose value. In someembodiments, the total signal can be filtered using a non-recursivefilter, such as a finite impulse response (FIR) filter. An FIR filter isa digital signal filter, in which every sample of output is the weightedsum of past and current samples of input, using only some finite numberof past samples. In some embodiments, the total signal can be filteredusing a recursive filter, such as an infinite impulse response (IIR)filter. An IIR filter is a type of digital signal filter, in which everysample of output is the weighted sum of past and current samples ofinput. In some embodiments, the total signal can be filtered using amaximum-average (max-average) filtering algorithm, which smoothes databased on the discovery that the substantial majority of signal artifactsobserved after implantation of glucose sensors in humans, for example,is not distributed evenly above and below the actual blood glucoselevels. It has been observed that many data sets are actuallycharacterized by extended periods in which the noise appears to trenddownwardly from maximum values with occasional high spikes. To overcomethese downward trending signal artifacts, the max-average calculationtracks with the highest sensor values, and discards the bulk of thelower values. Additionally, the max-average method is designed to reducethe contamination of the data with unphysiologically high data from thehigh spikes. The max-average calculation smoothes data at a samplinginterval (e.g., every 30 seconds) for transmission to the receiver at aless frequent transmission interval (e.g., every 5 minutes), to minimizethe effects of low non-physiological data. First, the microprocessorfinds and stores a maximum sensor counts value in a first set of sampleddata points (e.g., 5 consecutive, accepted, thirty-second data points).A frame shift time window finds a maximum sensor counts value for eachset of sampled data (e.g., each 5-point cycle length) and stores eachmaximum value. The microprocessor then computes a rolling average (e.g.,5-point average) of these maxima for each sampling interval (e.g., every30 seconds) and stores these data. Periodically (e.g., every 10^(th)interval), the sensor outputs to the receiver the current maximum of therolling average (e.g., over the last 10 thirty-second intervals as asmoothed value for that time period (e.g., 5 minutes)). In someembodiments, the total signal can be filtered using a ‘Cone ofPossibility Replacement Method,’ which utilizes physiologicalinformation along with glucose signal values in order define a ‘cone’ ofphysiologically feasible glucose signal values within a human.Particularly, physiological information depends upon the physiologicalparameters obtained from continuous studies in the literature as well asour own observations. A first physiological parameter uses a maximalsustained rate of change of glucose in humans (e.g., about 4 to 6mg/di/min) and a maximum sustained acceleration of that rate of change(e.g., about 0.1 to 0.2 mg/min/min). A second physiological parameteruses the knowledge that rate of change of glucose is lowest at themaxima and minima, which are the areas of greatest risk in patienttreatment. A third physiological parameter uses the fact that the bestsolution for the shape of the curve at any point along the curve over acertain time period (e.g., about 20-25 minutes) is a straight line. Themaximum rate of change can be narrowed in some instances. Therefore,additional physiological data can be used to modify the limits imposedupon the Cone of Possibility Replacement Method for sensor glucosevalues. For example, the maximum per minute rate of change can be lowerwhen the subject is lying down or sleeping; on the other hand, themaximum per minute rate change can be higher when the subject isexercising, for example. In some embodiments, the total signal can befiltered using reference changes in electrode potential to estimateglucose sensor data during positive detection of signal artifacts froman electrochemical glucose sensor, the method hereinafter referred to asreference drift replacement; in this embodiment, the electrochemicalglucose sensor comprises working, counter, and reference electrodes.This method exploits the function of the reference electrode as itdrifts to compensate for counter electrode limitations during oxygendeficits, pH changes, or temperature changes. In alternativeimplementations of the reference drift method, a variety of algorithmscan therefore be implemented based on the changes measured in thereference electrode. Linear algorithms, and the like, are suitable forinterpreting the direct relationship between reference electrode driftand the non-glucose rate limiting signal noise such that appropriateconversion to signal noise compensation can be derived. Additionaldescription of signal filtering can be found in U.S. Patent PublicationNo. US-2005-0043598-A1.

The constant noise signal component 1030 can be obtained by calibratingthe sensor signal using reference data, such as one or more bloodglucose values obtained from a hand-held blood glucose meter, or thelike, from which the baseline ‘b’ of a regression can be obtained,representing the constant noise signal component 1030.

The analyte signal component 1040 can be obtained by subtracting theconstant noise signal component 1030 from the filtered signal 1020.

In general, non-constant noise is caused by interfering species(non-constant noise-causing species), which can be compounds, such asdrugs that have been administered to the host, or intermittentlyproduced products of various host metabolic processes. Exemplaryinterferents include but are not limited to a variety of drugs (e.g.,acetaminophen), H₂O₂ from exterior sources (e.g., produced outside thesensor membrane system), and reactive metabolic species (e.g., reactiveoxygen and nitrogen species, some hormones, etc.). Some knowninterfering species for a glucose sensor include but are not limited toacetaminophen, ascorbic acid, bilirubin, cholesterol, creatinine,dopamine, ephedrine, ibuprofen, L-dopa, methyldopa, salicylate,tetracycline, tolazamide, tolbutamide, triglycerides, and uric acid.

In some experiments of implantable glucose sensors, it was observed thatnoise increased when some hosts were intermittently sedentary, such asduring sleep or sitting for extended periods. When the host began movingagain, the noise quickly dissipated. Noise that occurs duringintermittent, sedentary periods (also referred to as intermittentsedentary noise) can occur during relatively inactive periods, such assleeping. Non-constant, non-analyte-related factors can causeintermittent sedentary noise, such as was observed in one exemplarystudy of non-diabetic individuals implanted with enzymatic-type glucosesensors built without enzyme. These sensors (without enzyme) could notreact with or measure glucose and therefore provided a signal due tonon-glucose effects only (e.g., constant and non-constant noise). Duringsedentary periods (e.g., during sleep), extensive, sustained signal wasobserved on the sensors. Then, when the host got up and moved around,the signal rapidly corrected. As a control, in vitro experiments wereconducted to determine if a sensor component might have leached into thearea surrounding the sensor and caused the noise, but none was detected.From these results, it is believed that a host-produced non-analyterelated reactant was diffusing to the electrodes and producing theunexpected non-constant noise signal.

Interferents

Interferents are molecules or other species that may cause a sensor togenerate a false positive or negative analyte signal (e.g., anon-analyte-related signal). Some interferents are known to becomereduced or oxidized at the electrochemically reactive surfaces of thesensor, while other interferents are known to interfere with the abilityof the enzyme (e.g., glucose oxidase) used to react with the analytebeing measured. Yet other interferents are known to react with theenzyme (e.g., glucose oxidase) to produce a byproduct that iselectrochemically active. Interferents can exaggerate or mask theresponse signal, thereby leading to false or misleading results. Forexample, a false positive signal may cause the host's analyteconcentration (e.g., glucose concentration) to appear higher than thetrue analyte concentration. False-positive signals may pose a clinicallysignificant problem in some conventional sensors. For example in asevere hypoglycemic situation, in which the host has ingested aninterferent (e.g., acetaminophen), the resulting artificially highglucose signal can lead the host to believe that he is euglycemic orhyperglycemic. In response, the host may make inappropriate treatmentdecisions, such as by injecting himself with too much insulin, or bytaking no action, when the proper course of action would be to begineating. In turn, this inappropriate action or inaction may lead to adangerous hypoglycemic episode for the host. Accordingly, it is desiredthat a membrane system can be developed that substantially reduces oreliminates the effects of interferents on analyte measurements. Asdescribed in more detail elsewhere herein, it is contemplated that amembrane system having one or more domains capable of blocking orsubstantially reducing the flow of interferents onto the electroactivesurfaces of the electrode may reduce noise and improve sensor accuracy.

With respect to analyte sensors, it is contemplated that a number oftypes of interferents may cause inaccurate readings. One type ofinterferents is defined herein as ‘exogenous interferents.’ The term‘exogenous interferents’ as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and are not to be limited to a special or customizedmeaning), and refers without limitation to interferents that affect themeasurement of glucose and that are present in the host, but that haveorigins outside of the body, and that can include items administered toa person, such as medicaments, drugs, foods or herbs, whetheradministered intravenously, orally, topically, etc. By way of example,acetaminophen ingested by a host or the lidocaine injected into a hostwould be considered herein as exogenous interferents.

Another type of interferents is defined herein as ‘endogenousinterferents.’ The term ‘endogenous interferents’ as used herein is abroad term, and is to be given its ordinary and customary meaning to aperson of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refers without limitation tointerferents that affect the measurement of glucose and that haveorigins within the body, and thus includes interferents derived fromspecies or metabolites produced during cell metabolism (e.g., as aresult of wound healing). While not wishing to be bound by theory, it isbelieved that a local build up of electroactive interferents, such aselectroactive metabolites derived from cellular metabolism and woundhealing, may interfere with sensor function and cause earlyintermittent, sedentary noise. Local lymph pooling, when parts of thebody are compressed or when the body is inactive, may also cause, inpart, this local build up of interferents (e.g., electroactivemetabolites). Endogenous interferents may react with the membrane systemin ways that are different from exogenous interferents. Endogenousinterferents may include but are not limited to compounds withelectroactive acidic, amine or sulfhydryl groups, urea (e.g., as aresult of renal failure), lactic acid, phosphates, citrates, peroxides,amino acids (e.g., L-arginine), amino acid precursors or break-downproducts, nitric oxide (NO), NO-donors, NO-precursors, or otherelectroactive species or metabolites produced during cell metabolism orwound healing, for example.

Noise-Reducing Membrane System

In some embodiments, the continuous sensor may have a bioprotectivedomain which includes a polymer containing one or more surface-activegroups configured to substantially reduce or block the effect orinfluence of non-constant noise-causing species. In some of theseembodiments, the reduction or blocking of the effect or influence ofnon-constant noise-causing species may be such that the non-constantnoise component of the signal is less than about 60%, 50%, 40%, 30%,20%, or 10% of the total signal. In some embodiments, the sensor mayinclude at least one electrode and electronics configured to provide asignal measured at the electrode. The measured signal can be broken down(e.g., after sensor break-in) into its component parts, which mayinclude but are not limited to a substantially analyte-relatedcomponent, a substantially constant non-analyte-related component (e.g.,constant noise), and a substantially non-constant non-analyte-relatedcomponent (e.g., non-constant noise). In some of these embodiments, thesensor may be configured such that the substantially non-constantnon-analyte-related component does not substantially contribute to thesignal for at least about one or two days. In some embodiments, thesignal contribution of the non-constant noise may be less than about60%, 50%, 40%, 30%, 20%, or 10% of the signal (i.e., total signal) overa time period of at least about one day, but in other embodiments, thetime period may be at least about two, three, four, five, six, sevendays or more, including weeks or months, and the signal contribution ofthe non-constant noise may be less than about 18%, 16%, 14%, 12%, 10%,8%, 6%, 5%, 4%, 3%, 2%, or 1%. It is contemplated that in someembodiments, the sensor may be configured such that the signalcontribution of the analyte-related component is at least about 50%,60%, 70%, 80%, 90% or more of the total signal over a time period of atleast about one day; but in some embodiments, the time period may be atleast about two, three, four, five, six, seven days or more, includingweeks or months, and the signal contribution of the analyte-relatedcomponent may be at least about 50%, 55%, 60%, 65%, 70%, 75%, 80%, 82%,84%, 86%, 88%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more.

A signal component's percentage of the total signal can be determinedusing a variety of methods of quantifying an amplitude of signalcomponents and total signal, from which each component's percentcontribution can be calculated. In some embodiments, the signalcomponents can be quantified by comparing the peak-to-peak amplitudes ofeach signal component for a time period, whereby the peak-to-peakamplitudes of each component can be compared to the peak-to-peakamplitude of the total signal to determine its percentage of the totalsignal. In some embodiments, the signal components can be quantified bydetermining the Root Mean Square (RMS) of the signal component for atime period. In one exemplary of Root Mean Square analysis of signalcomponents, the signal component(s) can be quantified using the formula:

${RMS} = \sqrt{\frac{\sum\left( {x_{1}^{2} + x_{2}^{2} + x_{3}^{2} + x_{n}^{2}} \right)}{n}}$wherein there are a number (n) of data values (x) for a signal (e.g.,analyte component, non-constant noise component, constant noisecomponent, and total signal) during a predetermined time period (e.g.,about 1 day, about 2 days, about 3 days, etc). Once the signalcomponents and total signal are quantified, the signal components can becompared to the total signal to determine a percentage of each signalcomponent within the total signal.Bioprotective Domain

The bioprotective domain is the domain or layer of an implantable deviceconfigured to interface with (e.g., contact) a biological fluid whenimplanted in a host or connected to the host (e.g., via an intravascularaccess device providing extracorporeal access to a blood vessel). Asdescribed above, membranes of some embodiments may include abioprotective domain 46 (see FIGS. 2A-2C), also referred to as abioprotective layer, including at least one polymer containing asurface-active group. In some embodiments, the surface-activegroup-containing polymer is a surface-active end group-containingpolymer. In some of these embodiments, the surface-active endgroup-containing polymer is a polymer having covalently bondedsurface-active end groups. However, it is contemplated that othersurface-active group-containing polymers may also be used and can beformed by modification of fully-reacted base polymers via the graftingof side chain structures, surface treatments or coatings applied aftermembrane fabrication (e.g., via surface-modifying additives), blendingof a surface-modifying additive to a base polymer before membranefabrication, immobilization of the surface-active-group-containing softsegments by physical entrainment during synthesis, or the like.

Base polymers useful for certain embodiments may include any linear orbranched polymer on the backbone structure of the polymer. Suitable basepolymers may include, but are not limited to, epoxies, polyolefins,polysiloxanes, polyethers, acrylics, polyesters, carbonates, andpolyurethanes, wherein polyurethanes may include polyurethane copolymerssuch as polyether-urethane-urea, polycarbonate-urethane,polyether-urethane, silicone-polyether-urethane,silicone-polycarbonate-urethane, polyester-urethane, and the like. Insome embodiments, base polymers may be selected for their bulkproperties, such as, but not limited to, tensile strength, flex life,modulus, and the like. For example, polyurethanes are known to berelatively strong and to provide numerous reactive pathways, whichproperties may be advantageous as bulk properties for a membrane domainof the continuous sensor.

In some embodiments, a base polymer synthesized to have hydrophilicsegments may be used to form the bioprotective layer. For example, alinear base polymer including biocompatible segmented block polyurethanecopolymers comprising hard and soft segments may be used. In someembodiments, the hard segment of the copolymer may have a molecularweight of from about 160 daltons to about 10,000 daltons, and in certainembodiments from about 200 daltons to about 2,000 daltons. In someembodiments, the molecular weight of the soft segment may be from about200 daltons to about 10,000,000 daltons, and in certain embodiments fromabout 500 daltons to about 5,000,000 daltons, and in certain embodimentsfrom about 500,00 daltons to about 2,000,000 daltons. It is contemplatedthat polyisocyanates used for the preparation of the hard segments ofthe copolymer may be aromatic or aliphatic diisocyanates. The softsegments used in the preparation of the polyurethane may be apolyfunctional aliphatic polyol, a polyfunctional aliphatic or aromaticamine, or the like that may be useful for creating permeability of theanalyte (e.g., glucose) therethrough, and may include, for example,polyvinyl acetate (PVA), poly(ethylene glycol) (PEG), polyacrylamide,acetates, polyethylene oxide (PEO), polyethylacrylate (PEA),polyvinylpyrrolidone (PVP), variations thereof (e.g., PVP vinylacetate), and copolymers, mixtures, and/or combinations thereof (e.g., ablend of polyurethane-PVP vinyl acetate copolymer with a PVP polymer).

Alternatively, in some embodiments, the bioprotective layer may comprisea combination of a base polymer (e.g., polyurethane) and one or morehydrophilic polymers, such as, PVA, PEG, polyacrylamide, acetates, PEO,PEA, PVP, and variations thereof (e.g., PVP vinyl acetate), e.g., as aphysical blend or admixture wherein each polymer maintains its uniquechemical nature. It is contemplated that any of a variety of combinationof polymers may be used to yield a blend with desired glucose, oxygen,and interference permeability properties. For example, in someembodiments, the bioprotective layer may be formed from a blend of apolycarbonate-urethane base polymer and PVP, but in other embodiments, ablend of a polyurethane, or another base polymer, and one or morehydrophilic polymers may be used instead. In some of the embodimentsinvolving use of PVP, the PVP portion of the polymer blend may comprisefrom about 5% to about 50% by weight of the polymer blend, in certainembodiments from about 15% to 20%, and in other embodiments from about25% to 40%. It is contemplated that PVP of various molecular weights maybe used. For example, in some embodiments, the molecular weight of thePVP used may be from about 25,000 daltons to about 5,000,000 daltons, incertain embodiments from about 50,000 daltons to about 2,000,000daltons, and in other embodiments from 6,000,000 daltons to about10,000,000 daltons. In still other embodiments, the bioprotective layermay comprise a combination of a base polymer having one or morehydrophilic segments and a hydrophilic polymer. The hydrophilic segmentsof the base polymer and the hydrophilic polymer may include, but are notlimited to, polyvinyl acetate (PVA), poly(ethylene glycol) (PEG),polyacrylamide, acetates, polyethylene oxide (PEO), poly ethyl acrylate(PEA), polyvinylpyrrolidone, and copolymers, variations, andcombinations thereof (e.g., PVP vinyl acetate).

Membranes have been developed that are capable of controlling the fluxof a particular analyte passing through the membrane. However, it isknown that conventional membranes typically lack the capability ofsubstantially reducing or blocking the flux of interferents passingtherethrough. From a membrane design perspective, typically as amembrane is made more permeable (i.e., opened up) for an analyte to passthrough, this increased permeability of the membrane for the analytetends to also increase the permeability of interferents. As an example,a conventional membrane that allows for a flux of glucose (with a M.W.of 180 daltons) through the membrane will typically not substantiallyreduce or block the flux of interferents, such as acetaminophen (with aM.W. of 151.2 daltons) through the membrane. Accordingly, without amechanism designed to reduce the flux of interferents, large levels ofundesirable signal noise may be generated as a result of theinterferents passing through the membrane. Advantageously, someembodiments described herein provide a membrane layer that overcomes theabove-described deficiencies by providing a mechanism for selectivelycontrolling the flux of a particular analyte, while also substantiallyreducing or blocking the flux of interferents through the membrane.

While not wishing to be bound by theory, it is believed that in someconventional membranes formed with various segmented block polyurethanecopolymers, the hydrophobic portions of the copolymer (e.g., the hardsegments) may tend to segregate from the hydrophilic portions (e.g., thesoft segments), which in turn, may cause the hydrophilic portions toalign and form channels, through which analytes, such as glucose, andother molecules, such as exogenous interferents like acetaminophen, maypass through the bioprotective layer from the distal surface to theproximal surface. While the diffusion of analytes through thebioprotective layer is desired, the diffusion of interferents isgenerally not. Through experiments, it has been unexpectedly found thatthe use of PVP blended with a base polymer, such as,silicone-polycarbonate-urethane, may provide the bioprotective layerwith the capability of substantially reducing or blocking the flux ofvarious interferents, such as acetaminophen, through the layer. Whilenot wishing to be bound by theory, it is believed that the carbonylgroups of PVP molecules may form hydrogen bonds with variousinterferents. For example, acetaminophen molecules are known to becapable of hydrogen bonding via their hydroxyl (O—H) and amide(H—N—(C═O)) groups, and thus through these moieties may interact withPVP. Although PVP is described here to provide an example of ahydrophilic polymer capable of providing the hydrogen bonding effectsdescribed above, it is contemplated that any of a variety of otherhydrophilic polymers known to have strong hydrogen bonding propertiesmay also be used, such as, polyvinyl pyrrolidone-vinyl acetate (PVP-VA),hydroxypropyl cellulose (HPC), hydroxypropyl methylcellulose (HPMC), forexample.

In some embodiments, the bioprotective domain is configured tosubstantially reduce or block the flux of at least one interferent, andexhibits a glucose-to-interferent permeability ratio of approximately 1to 30, but in other embodiments the glucose-to-interferent permeabilityratio (e.g., glucose-to-acetaminophen permeability ratio) may be lessthan approximately 1 to 1, 1 to 2, 1 to 5, 1 to 10, 1 to 15, 1 to 20, 1to 35, 1 to 40, 1 to 45, 1 to 50, or 1 to 100. Theglucose-to-interferent permeability ratios exhibited by theseembodiments are an improvement over conventional polyurethane membraneswhich typically exhibit glucose-to-interferent permeability ratios(e.g., glucose-to-acetaminophen permeability ratios) greater than 1 to300. In some embodiments, the equivalent peak glucose response to a1,000 mg dose of acetaminophen is less than about 100 mg/dL, in certainembodiments less than 80 mg/dL, and in other embodiments less than about50 mg/dL, and in still other embodiments less than about 20 mg/dL.

FIG. 8 illustrates and Example 5 describes the level of blocking of theinterferent acetaminophen as exhibited by a bioprotective domaincomprising PVP blended with silicone-polycarbonate-urethane basepolymer. While this particular polymer was formed by blending a basesilicone-polycarbonate-urethane polymer with PVP before membranefabrication, it is contemplated that other methods, such as, surfacetreatments applied after membrane fabrication (e.g., viasurface-modifying additives), immobilization ofsurface-active-group-containing segments by physical entrainment duringsynthesis of the polymer, for example, may also be used and may alsoprovide similar results.

In some embodiments, the PVP portion of the polymer blend may comprisefrom about 5% to about 50% by weight of the polymer blend, in certainembodiments from about 15% to 20%, and in other embodiments from about25% to 40%. It is contemplated that PVP of various molecular weights maybe used. For example, in some embodiments, the molecular weight of thePVP used may be from about 25,000 daltons to about 5,000,000 daltons, incertain embodiments from about 50,000 daltons to about 2,000,000daltons, and in other embodiments from 6,000,000 daltons to about10,000,000 daltons.

The term ‘surface-active group’ and ‘surface-active end group’ as usedherein are broad terms and are used in their ordinary sense, including,without limitation, surface-active oligomers or other surface-activemoieties having surface-active properties, such as alkyl groups, whichpreferentially migrate towards a surface of a membrane formed therefrom. Surface-active groups preferentially migrate toward air (e.g.,driven by thermodynamic properties during membrane formation). In someembodiments, the surface-active groups are covalently bonded to the basepolymer during synthesis. In some preferred embodiments, surface-activegroups may include silicone, sulfonate, fluorine, polyethylene oxide,hydrocarbon groups, and the like. The surface activity (e.g., chemistry,properties) of a membrane domain including a surface-activegroup-containing polymer reflects the surface activity of thesurface-active groups rather than that of the base polymer. In otherwords, surface-active groups control the chemistry at the surface (e.g.,the biological contacting surface) of the membrane without compromisingthe bulk properties of the base polymer. The surface-active groups ofthe preferred embodiments are selected for desirable surface properties,for example, non-constant noise-blocking ability, break-in time(reduced), ability to repel charged species, cationic or anionicblocking, or the like. In some preferred embodiments, the surface-activegroups are located on one or more ends of the polymer backbone, andreferred to as surface-active end groups, wherein the surface-active endgroups are believed to more readily migrate to the surface of thebioprotective domain/layer formed from the surface-activegroup-containing polymer in some circumstances.

FIG. 4A is a schematic view of a base polymer 400 having surface-activeend groups in one embodiment. In some preferred embodiments, thesurface-active moieties 402 are restricted to the termini of the linearor branched base polymer(s) 400 such that changes to the base polymer'sbulk properties are minimized. Because the polymers couple end groups tothe backbone polymer during synthesis, the polymer backbone retains itsstrength and processability. The utility of surface-active end groups isbased on their ability to accumulate at the surface of a formed articlemade from the surface-active end group-containing polymer. Suchaccumulation is driven by the minimization of interfacial energy of thesystem, which occurs as a result of it.

FIG. 4B is a schematic view of a bioprotective domain, showing aninterface in a biological environment (e.g., interstitial space orvascular space). The preferred surface-active group-containing polymeris shown fabricated as a membrane 46, wherein the surface-active endgroups have migrated to the surface of the base polymer. While notwishing to be bound by theory, it is believed that this surface isdeveloped by surface-energy-reducing migrations of the surface-activeend groups to the air-facing surface during membrane fabrication. It isalso believed that the hydrophobicity and mobility of the end groupsrelative to backbone groups facilitate the formation of this uniformover layer by the surface-active (end) blocks.

In some embodiments, the bioprotective domain 46 is formed from apolymer containing silicone as the surface-active group, for example, apolyurethane containing silicone end group(s). Some embodiments includea continuous analyte sensor configured for insertion into a host,wherein the sensor has a membrane located over the sensing mechanism,which includes a polyurethane comprising silicone end groups configuredto substantially block the effect of non-constant noise-causing specieson the sensor signal, as described in more detail elsewhere herein. Insome embodiments, the polymer includes about 10%, 11%, 12%, 13%, 14%,15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%,29%, 30%, to about 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%,41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54% or55% silicone by weight. In certain embodiments, the silicone (e.g., aprecursor such as PDMS) has a molecular weight from about 500 to about10,000 daltons, preferably at least about 200 daltons. In someembodiments, the base polymer includes at least about 10% silicone byweight, and preferably from about 19% to about 40% silicone by weight.These ranges are believed to provide an advantageous balance ofnoise-reducing functionality, while maintaining sufficient glucosepermeability in embodiments wherein the sensor is a glucose sensor, forexample.

In some embodiments, the bioprotective domain is formed from a polymercontaining fluorine as a surface-active group, for example, apolyurethane that contains a fluorine end groups. In preferredembodiments, the polymer includes from about 1% to about 25% fluorine byweight. Some embodiments include a continuous analyte sensor configuredfor insertion into a host, wherein the sensor has a membrane locatedover the sensing mechanism, wherein the membrane includes a polyurethanecontaining fluorine surface-active groups, and wherein the membrane isconfigured and arranged to reduce a break-in time of a sensor ascompared to a membrane formed from a similar base polymer without thesurface-active group(s). For example, in preferred embodiments, aglucose sensor having a bioprotective domain of the preferredembodiments has a response time (e.g., t₉₀) of less than 120 seconds, incertain embodiments less than 60 seconds, and in still other embodimentsless than about 45, 30, 20, or 10 seconds (across a physiological rangeof glucose concentration).

In some embodiments, the bioprotective domain may be formed from apolymer that contains sulfonate as a surface-active group, for example,a polyurethane containing sulfonate end group(s). In some embodiments,the continuous analyte sensor configured for insertion into a host mayinclude a membrane located over the sensing mechanism, wherein themembrane includes a polymer that contains sulfonate as a surface-activegroup, and is configured to repel charged species, for example, due tothe net negative charge of the sulfonated groups.

In some embodiments, a blend of two or more (e.g., two, three, four,five, or more) surface-active group-containing polymers is used to forma bioprotective membrane domain. For example, by blending a polyurethanewith silicone end groups and a polyurethane with fluorine end groups,and forming a bioprotective membrane domain from that blend, a sensorcan be configured to substantially block non-constant noise-causingspecies and reduce the sensor's t₉₀, as described in more detailelsewhere herein. Similarly, by blending a polyurethane containingsilicone end groups, a polyurethane containing fluorine end groups, anda polyurethane containing sulfonate end groups, and forming abioprotective membrane domain from that blend, a sensor can beconfigured to substantially block non-constant noise-causing species, toreduce the sensor's break-in time and to repel charged species, asdescribed in more detail above. Although in some embodiments, blendingof two or more surface-active group-containing polymers is used, inother embodiments, a single component polymer can be formed bysynthesizing two or more surface-active groups with a base polymer toachieve similarly advantageous surface properties; however, blending maybe preferred in some embodiments for ease of manufacture.

As described in Example 8 below, in some embodiments, sensors employinga bioprotective domain have not only demonstrated greater levels ofoverall accuracy, but also greater levels of accuracy at low glucoseconcentration levels (e.g., at glucose concentration levels from about40 mg/dL to about 80 mg/dL), as compared to conventional continuousglucose sensors. The ability of certain sensor embodiments,incorporating a bioprotective domain, to measure accurately at lowglucose concentration levels can not only be valuable, but at times canbe critical to the user of the device. Whereas there is typically littleimmediate danger from hyperglycemia, there can be very real immediatedanger from hypoglycemia. Severe hypoglycemia can lead to mentaldisorientation, unconsciousness, seizure, accidents, physical injury,and sometimes death. Thus, while there can be a large level of clinicaltolerance for sensor errors at the euglycemic range and even at thehyperglycemic range, the tolerance level at low glucose concentrationlevels is generally much lower. Accordingly, it may be desirable toprovide a sensor that can meet higher accuracy standards, particularlyin the hypoglycemic range, in order to provide accurate feedback forappropriate and timely treatment decision.

Conventional sensors typically do not perform as accurately in thehypoglycemic range as they do in higher glucose concentration ranges.When a sensor is calibrated, the sensor is generally calibrated acrossan entire glucose concentration range. As a result, because theconversion function, as derived from the calibration and used to convertsignal amplitude (counts) to glucose concentration, may contain error(e.g., because of inaccurate calibration or because of imperfection inthe ideal linear relationship between the signal amplitude and actualglucose concentration), an inherent measurement inaccuracy may exist atany glucose concentration level. What's more, errors at low glucoseconcentrations can often be magnified, as compared to errors at otherglucose concentration ranges. Accordingly, a sensor's overall accuracy(e.g., as measured in terms of mean absolute relative difference) istypically not representative of the sensor's accuracy at hypoglycemiclevels, and in fact, is typically less accurate at these levels. Whilenot wishing to be bound by theory, it is believed that this phenomenonoccurs with conventional sensors in part because at low glucoseconcentrations a smaller glucose signal amplitude is generated (ascompared to the signal amplitude associated with normal or high glucoseconcentrations), while the baseline signal (or background or constantnoise) remain substantially constant and thus can in certaincircumstances proportionally overwhelm the smaller glucose signalamplitude. Thus, the glucose-signal-to-baseline-signal ratio at a lowglucose concentration is typically less than theglucose-signal-to-baseline-signal ratio at a high glucose concentration.

While not wishing to be bound by theory, it is believed that in certaintypes of sensors, accuracy in the hypoglycemic range may be improved byreducing the level of baseline signal without altering the sensitivity.This minimization of the baseline signal provides a gain in theglucose-signal-to-baseline-signal ratio, particularly at thehypoglycemic range. In turn, ceteris paribus, a much more accuraterepresentation of the glucose signal is obtained, as the effect of thebackground signal, which may contain inaccuracies for the reasonsdescribed above, on the total signal is reduced. By way of example, a 5%error in the baseline signal will typically cause a higher level ofinaccuracy and signal distortion in a sensor with a lowglucose-signal-to-baseline-signal ratio than that of a sensor with ahigh glucose-signal-to-baseline-signal.

A comparison of FIGS. 10A with 10B (both of which are not necessarilydrawn to scale) further illustrates this phenomenon. FIG. 10A displaysthe conversion function of a sensor with a high background signal, whileFIG. 10B displays the conversion function of a sensor similar to thesensor associated with FIG. 10A, but with a low background signal. Asillustrated, the sensitivities (i.e., the slopes of the conversionfunction as measured in units of mg/dLpA) of the two sensors are thesame. While not wishing to be bound by theory, it is believed that thesensor associated with FIG. 10B, ceteris paribus, is capable ofachieving better overall accuracy (and particularly better accuracy atthe hypoglycemic range) than the sensor associated with FIG. 10A,because of its better glucose-signal-to-baseline-signal ratio. As can berealized from comparing FIG. 10A with FIG. 10B, the difference in theglucose-signal-to-baseline-signal ratios, between the two sensors, isparticularly pronounced in the hypoglycemic range, and less so in theeuglycemic range, and even less so in the hyperglycemic range.Accordingly, while not wishing to be bound by theory, it is believedthat sensor accuracy improvement resulting from a reduction in baselinesignal may in certain circumstances be greater at the hypoglycemic rangethan at other ranges of higher glucose concentration levels.

In some embodiments, sensor accuracy may be improved by using a membranewith a bioprotective layer that unexpectedly and substantially reducesthe baseline signal, thereby providing not only better overall accuracy,but also better accuracy at the hypoglycemic range.

Membrane Fabrication

Preferably, polymers of the preferred embodiments may be processed bysolution-based techniques such as spraying, dipping, casting,electrospinning, vapor deposition, spin coating, coating, and the like.Water-based polymer emulsions can be fabricated to form membranes bymethods similar to those used for solvent-based materials. In both casesthe evaporation of a volatile liquid (e.g. organic solvent or water)leaves behind a film of the polymer. Cross-linking of the deposited filmmay be performed through the use of multi-functional reactiveingredients by a number of methods well known to those skilled in theart. The liquid system may cure by heat, moisture, high-energyradiation, ultraviolet light, or by completing the reaction, whichproduces the final polymer in a mold or on a substrate to be coated.

Domains that include at least two surface-active group-containingpolymers may be made using any of the methods of forming polymer blendsknown in the art. In one exemplary embodiment, a solution of apolyurethane containing silicone end groups is mixed with a solution ofa polyurethane containing fluorine end groups (e.g., wherein thesolutions include the polymer dissolved in a suitable solvent such asacetone, ethyl alcohol, DMAC, THF, 2-butanone, and the like). Themixture can then be drawn into a film or applied to a surface using anymethod known in the art (e.g., spraying, painting, dip coating, vapordepositing, molding, 3-D printing, lithographic techniques (e.g.,photolithograph), micro- and nano-pipetting printing techniques, etc.).The mixture can then be cured under high temperature (e.g., 50-150° C.).Other suitable curing methods may include ultraviolet or gammaradiation, for example.

Some amount of cross-linking agent can also be included in the mixtureto induce cross-linking between polymer molecules. Non-limiting examplesof suitable cross-linking agents include isocyanate, carbodiimide,gluteraldehyde or other aldehydes, epoxy, acrylates, free-radical basedagents, ethylene glycol diglycidyl ether (EGDE), poly(ethylene glycol)diglycidyl ether (PEGDE), or dicumyl peroxide (DCP). In one embodiment,from about 0.1% to about 15% w/w of cross-linking agent is addedrelative to the total dry weights of cross-linking agent and polymersadded when blending the ingredients (in one example, about 1% to about10%). During the curing process, substantially all of the cross-linkingagent is believed to react, leaving substantially no detectableunreacted cross-linking agent in the final film.

In some embodiments, the bioprotective domain 46 is positioned mostdistally to the sensing region such that its outer most domain contactsa biological fluid when inserted in vivo. In some embodiments, thebioprotective domain is resistant to cellular attachment, impermeable tocells, and may be composed of a biostable material. While not wishing tobe bound by theory, it is believed that when the bioprotective domain 46is resistant to cellular attachment (for example, attachment byinflammatory cells, such as macrophages, which are therefore kept asufficient distance from other domains, for example, the enzyme domain),hypochlorite and other oxidizing species are short-lived chemicalspecies in vivo, and biodegradation does not generally occur.Additionally, the materials preferred for forming the bioprotectivedomain 46 may be resistant to the effects of these oxidative species andhave thus been termed biodurable. In some embodiments, the bioprotectivedomain controls the flux of oxygen and other analytes (for example,glucose) to the underlying enzyme domain (e.g., wherein thefunctionality of the diffusion resistance domain is built-into thebioprotective domain such that a separate diffusion resistance domain isnot required).

In certain embodiments, the thickness of the bioprotective domain may befrom about 0.1, 0.5, 1, 2, 4, 6, 8 microns or less to about 10, 15, 20,30, 40, 50, 75, 100, 125, 150, 175, 200 or 250 microns or more. In someof these embodiments, the thickness of the bioprotective domain may bein certain embodiments from about 1 to about 5 microns, and in otherembodiments from about 2 to about 7 microns. In other embodiments, thebioprotective domain may be from about 20 or 25 microns to about 50, 55,or 60 microns thick. In some embodiments, the glucose sensor may beconfigured for transcutaneous or short-term subcutaneous implantation,and may have a thickness from about 0.5 microns to about 8 microns, orin certain embodiments from about 4 microns to about 6 microns. In oneglucose sensor configured for fluid communication with a host'scirculatory system, the thickness may be from about 1.5 microns to about25 microns, and in certain embodiments from about 3 to about 15 microns.It is also contemplated that in some embodiments, the bioprotectivelayer or any other layer of the electrode may have a thickness that isconsistent, but in other embodiments, the thickness may vary. Forexample, in some embodiments, the thickness of the bioprotective layermay vary along the longitudinal axis of the electrode end.

Diffusion Resistance Domain

In some embodiments, a diffusion resistance domain 44, also referred toas a diffusion resistance layer, may be used and is situated moreproximal to the implantable device relative to the bioprotective domain.In some embodiments, the functionality of the diffusion resistancedomain may be built into the bioprotective domain that comprises thesurface-active group-containing base polymer. Accordingly, thedescription herein of the diffusion resistance domain may also apply tothe bioprotective domain. The diffusion resistance domain serves tocontrol the flux of oxygen and other analytes (for example, glucose) tothe underlying enzyme domain. There typically exists a molar excess ofglucose in a body relative to the amount of oxygen in interstitial fluidor blood, e.g., for every free oxygen molecule in extracellular fluid,there are typically more than 100 glucose molecules present (see Updikeet al., Diabetes Care 5:207-21 (1982)). To achieve accurate sensormeasurements of glucose concentration, the amount of oxygen present forthe glucose-oxidase-catalyzed reaction has to be greater than that ofglucose. Otherwise, an oxygen limiting reaction, instead of a glucoselimiting reaction, may occur, especially in high glucose concentrationlevels. More specifically, when a glucose-monitoring reaction isoxygen-limited, linearity is not achieved above minimal concentrationsof glucose. Without a semipermeable membrane situated over the enzymedomain to control the flux of glucose and oxygen, a linear response toglucose levels can be obtained only up to about 40 mg/dL. However, in aclinical setting, a linear response to glucose levels is desirable up toat least about 500 mg/dL.

The diffusion resistance domain 44 includes a semipermeable membranethat controls the flux of oxygen and glucose to the underlying enzymedomain 44, preferably rendering oxygen in non-rate-limiting excess. As aresult, the upper limit of linearity of glucose measurement is extendedto a much higher value than that which is achieved without the diffusionresistance domain. In some embodiments, the diffusion resistance domainexhibits an oxygen-to-glucose permeability ratio of approximately 200:1,but in other embodiments the oxygen-to-glucose permeability ratio may beapproximately 100:1, 125:1, 130:1, 135:1, 150:1, 175:1, 225:1, 250:1,275:1, 300:1, or 500:1. As a result of the high oxygen-to-glucosepermeability ratio, one-dimensional reactant diffusion may providesufficient excess oxygen at all reasonable glucose and oxygenconcentrations found in the subcutaneous matrix (See Rhodes et al.,Anal. Chem., 66:1520-1529 (1994)). In some embodiments, a lower ratio ofoxygen-to-glucose can be sufficient to provide excess oxygen by using ahigh oxygen soluble domain (for example, a silicone material) to enhancethe supply/transport of oxygen to the enzyme membrane or electroactivesurfaces. By enhancing the oxygen supply through the use of a siliconecomposition, for example, glucose concentration can be less of alimiting factor. In other words, if more oxygen is supplied to theenzyme or electroactive surfaces, then more glucose can also be suppliedto the enzyme without creating an oxygen rate-limiting excess.

In some embodiments, the diffusion resistance domain is formed of a basepolymer synthesized to include a polyurethane membrane with bothhydrophilic and hydrophobic regions to control the diffusion of glucoseand oxygen to an analyte sensor. A suitable hydrophobic polymercomponent may be a polyurethane or polyether urethane urea. Polyurethaneis a polymer produced by the condensation reaction of a diisocyanate anda difunctional hydroxyl-containing material. A polyurea is a polymerproduced by the condensation reaction of a diisocyanate and adifunctional amine-containing material. Preferred diisocyanates includealiphatic diisocyanates containing from about 4 to about 8 methyleneunits. Diisocyanates containing cycloaliphatic moieties can also beuseful in the preparation of the polymer and copolymer components of themembranes of preferred embodiments. The material that forms the basis ofthe hydrophobic matrix of the diffusion resistance domain can be any ofthose known in the art as appropriate for use as membranes in sensordevices and as having sufficient permeability to allow relevantcompounds to pass through it, for example, to allow an oxygen moleculeto pass through the membrane from the sample under examination in orderto reach the active enzyme or electrochemical electrodes. Examples ofmaterials which can be used to make non-polyurethane type membranesinclude vinyl polymers, polyethers, polyesters, polyamides, inorganicpolymers such as polysiloxanes and polycarbosiloxanes, natural polymerssuch as cellulosic and protein based materials, and mixtures orcombinations thereof.

In one embodiment of a polyurethane-based resistance domain, thehydrophilic polymer component is polyethylene oxide. For example, oneuseful hydrophilic copolymer component is a polyurethane polymer thatincludes about 20% hydrophilic polyethylene oxide. The polyethyleneoxide portions of the copolymer are thermodynamically driven to separatefrom the hydrophobic portions of the copolymer and the hydrophobicpolymer component. The 20% polyethylene oxide-based soft segment portionof the copolymer used to form the final blend affects the water pick-upand subsequent glucose permeability of the membrane.

Alternatively, in some embodiments, the resistance domain may comprise acombination of a base polymer (e.g., polyurethane) and one or morehydrophilic polymers (e.g., PVA, PEG, polyacrylamide, acetates, PEO,PEA, PVP, and variations thereof). It is contemplated that any of avariety of combination of polymers may be used to yield a blend withdesired glucose, oxygen, and interference permeability properties. Forexample, in some embodiments, the resistance domain may be formed from ablend of a silicone polycarbonate-urethane base polymer and a PVPhydrophilic polymer, but in other embodiments, a blend of apolyurethane, or another base polymer, and one or more hydrophilicpolymers may be used instead. In some of the embodiments involving theuse of PVP, the PVP portion of the polymer blend may comprise from about5% to about 50% by weight of the polymer blend, in certain embodimentsfrom about 15% to 20%, and in other embodiments from about 25% to 40%.It is contemplated that PVP of various molecular weights may be used.For example, in some embodiments, the molecular weight of the PVP usedmay be from about 25,000 daltons to about 5,000,000 daltons, in certainembodiments from about 50,000 daltons to about 2,000,000 daltons, and inother embodiments from 6,000,000 daltons to about 10,000,000 daltons.

In some embodiments, the diffusion resistance domain 44 can be formed asa unitary structure with the bioprotective domain 46; that is, theinherent properties of the diffusion resistance domain 44 areincorporated into bioprotective domain 46 such that the bioprotectivedomain 46 functions as a diffusion resistance domain 44.

In certain embodiments, the thickness of the resistance domain may befrom about 0.05 microns or less to about 200 microns or more. In some ofthese embodiments, the thickness of the resistance domain may be fromabout 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2,2.5, 3, 3.5, 4, 6, 8 microns to about 9, 10, 11, 12, 13, 14, 15, 16, 17,18, 19, 19.5, 20, 30, 40, 50, 60, 70, 75, 80, 85, 90, 95, or 100microns. In some embodiments, the thickness of the resistance domain isfrom about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns in thecase of a transcutaneously implanted sensor or from about 20 or 25microns to about 40 or 50 microns in the case of a wholly implantedsensor.

Enzyme Domain

In some embodiments, an enzyme domain 42, also referred to as the enzymelayer, may be used and is situated less distal from theelectrochemically reactive surfaces than the diffusion resistance domain44. The enzyme domain comprises a catalyst configured to react with ananalyte. In one embodiment, the enzyme domain is an immobilized enzymedomain 42 including glucose oxidase. In other embodiments, the enzymedomain 42 can be impregnated with other oxidases, for example, galactoseoxidase, cholesterol oxidase, amino acid oxidase, alcohol oxidase,lactate oxidase, or uricase. For example, for an enzyme-basedelectrochemical glucose sensor to perform well, the sensor's responseshould neither be limited by enzyme activity nor cofactor concentration.

In some embodiments, the catalyst (enzyme) can be impregnated orotherwise immobilized into the bioprotective or diffusion resistancedomain such that a separate enzyme domain 42 is not required (e.g.,wherein a unitary domain is provided including the functionality of thebioprotective domain, diffusion resistance domain, and enzyme domain).In some embodiments, the enzyme domain 42 is formed from a polyurethane,for example, aqueous dispersions of colloidal polyurethane polymersincluding the enzyme.

In some embodiments, the thickness of the enzyme domain may be fromabout 0.01, 0.05, 0.6, 0.7, or 0.8 microns to about 1, 1.2, 1.4, 1.5,1.6, 1.8, 2, 2.1, 2.2, 2.5, 3, 4, 5, 10, 20, 30 40, 50, 60, 70, 80, 90,or 100 microns. In more preferred embodiments, the thickness of theenzyme domain is from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4,0.45, 0.5, 1, 1.5, 2, 2.5, 3, 4, or 5 microns to about 6, 7, 8, 9, 10,11, 12, 13, 14, 15, 16, 17, 18, 19, 19.5, 20, 25, or 30 microns. In evenmore preferred embodiments, the thickness of the enzyme domain is fromabout 2, 2.5, or 3 microns to about 3.5, 4, 4.5, or 5 microns in thecase of a transcutaneously implanted sensor or from about 6, 7, or 8microns to about 9, 10, 11, or 12 microns in the case of a whollyimplanted sensor.

Interference Domain

It is contemplated that in some embodiments, such as the embodimentillustrated in FIG. 2B, an optional interference domain 40, alsoreferred to as the interference layer, may be provided, in addition tothe bioprotective domain and the enzyme domain. The interference domain40 may substantially reduce the permeation of one or more interferentsinto the electrochemically reactive surfaces. Preferably, theinterference domain 40 is configured to be much less permeable to one ormore of the interferents than to the measured species. It is alsocontemplated that in some embodiments, where interferent blocking may beprovided by the bioprotective domain (e.g., via a surface-activegroup-containing polymer of the bioprotective domain), a separateinterference domain may not be used.

In some embodiments, the interference domain is formed from asilicone-containing polymer, such as a polyurethane containing silicone,or a silicone polymer. While not wishing to be bound by theory, it isbelieved that, in order for an enzyme-based glucose sensor to functionproperly, glucose would not have to permeate the interference layer,where the interference domain is located more proximal to theelectroactive surfaces than the enzyme domain. Accordingly, in someembodiments, a silicone-containing interference domain, comprising agreater percentage of silicone by weight than the bioprotective domain,may be used without substantially affecting glucose concentrationmeasurements. For example, in some embodiments, the silicone-containinginterference domain may comprise a polymer with a high percentage ofsilicone (e.g., from about 25%, 30%, 35%, 40%, 45%, or 50% to about 60%,70%, 80%, 90% or 95%).

In one embodiment, the interference domain may include ionic componentsincorporated into a polymeric matrix to reduce the permeability of theinterference domain to ionic interferents having the same charge as theionic components. In another embodiment, the interference domain mayinclude a catalyst (for example, peroxidase) for catalyzing a reactionthat removes interferents. U.S. Pat. No. 6,413,396 and U.S. Pat. No.6,565,509 disclose methods and materials for eliminating interferingspecies.

In certain embodiments, the interference domain may include a thinmembrane that is designed to limit diffusion of certain species, forexample, those greater than 34 kD in molecular weight. In theseembodiments, the interference domain permits certain substances (forexample, hydrogen peroxide) that are to be measured by the electrodes topass through, and prevents passage of other substances, such aspotentially interfering substances. In one embodiment, the interferencedomain is constructed of polyurethane. In an alternative embodiment, theinterference domain comprises a high oxygen soluble polymer, such assilicone.

In some embodiments, the interference domain is formed from one or morecellulosic derivatives. In general, cellulosic derivatives may includepolymers such as cellulose acetate, cellulose acetate butyrate,2-hydroxyethyl cellulose, cellulose acetate phthalate, cellulose acetatepropionate, cellulose acetate trimellitate, or blends and combinationsthereof.

In some alternative embodiments, other polymer types that can beutilized as a base material for the interference domain includepolyurethanes, polymers having pendant ionic groups, and polymers havingcontrolled pore size, for example. In one such alternative embodiment,the interference domain includes a thin, hydrophobic membrane that isnon-swellable and restricts diffusion of low molecular weight species.The interference domain is permeable to relatively low molecular weightsubstances, such as hydrogen peroxide, but restricts the passage ofhigher molecular weight substances, including glucose and ascorbic acid.Other systems and methods for reducing or eliminating interferencespecies that can be applied to the membrane system of the preferredembodiments are described in U.S. Pat. No. 7,074,307, U.S. PatentPublication No. US-2005-0176136-A1, U.S. Pat. No. 7,081,195, and U.S.Patent Publication No. US-2005-0143635-A1, each of which is incorporatedby reference herein in its entirety.

It is contemplated that in some embodiments, the thickness of theinterference domain may be from about 0.01 microns or less to about 20microns or more. In some of these embodiments, the thickness of theinterference domain may be from about 0.01, 0.05, 0.1, 0.15, 0.2, 0.25,0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4,5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns.In some of these embodiments, the thickness of the interference domainmay be from about 0.2, 0.4, 0.5, or 0.6, microns to about 0.8, 0.9, 1,1.5, 2, 3, or 4 microns.

In general, the membrane system may be formed or deposited on theexposed electroactive surfaces (e.g., one or more of the working andreference electrodes) using known thin film techniques (for example,casting, spray coating, drawing down, electro-depositing, dip coating,and the like), however casting or other known application techniques canalso be utilized. In some embodiments, the interference domain may bedeposited by spray or dip coating. In one exemplary embodiment, theinterference domain is formed by dip coating the sensor into aninterference domain solution using an insertion rate of from about 0.5inch/min to about 60 inches/min, and in certain embodiments about 1inch/min; a dwell time of from about 0.01 minutes to about 2 minutes,and in certain embodiments about 1 minute; and a withdrawal rate of fromabout 0.5 inch/minute to about 60 inches/minute, and in certainembodiments about 1 inch/minute; and curing (drying) the domain fromabout 1 minute to about 14 hours, and in certain embodiments from about3 minutes to about 15 minutes (and can be accomplished at roomtemperature or under vacuum (e.g., 20 to 30 mmHg)). In one exemplaryembodiment including a cellulose acetate butyrate interference domain, a3-minute cure (i.e., dry) time is used between each layer applied. Inanother exemplary embodiment employing a cellulose acetate interferencedomain, a 15 minute cure time is used between each layer applied.

In some embodiments, the dip process can be repeated at least one timeand up to 10 times or more. In other embodiments, only one dip ispreferred. The preferred number of repeated dip processes may dependupon the cellulosic derivative(s) used, their concentration, conditionsduring deposition (e.g., dipping) and the desired thickness (e.g.,sufficient thickness to provide functional blocking of certaininterferents), and the like. In one embodiment, an interference domainis formed from three layers of cellulose acetate butyrate. In anotherembodiment, an interference domain is formed from 10 layers of celluloseacetate. In yet another embodiment, an interference domain is formedfrom 1 layer of a blend of cellulose acetate and cellulose acetatebutyrate. In alternative embodiments, the interference domain can beformed using any known method and combination of cellulose acetate andcellulose acetate butyrate, as will be appreciated by one skilled in theart.

Electrode Domain

It is contemplated that in some embodiments, such as the embodimentillustrated in FIG. 2C, an optional electrode domain 36, also referredto as the electrode layer, may be provided, in addition to thebioprotective domain and the enzyme domain; however, in otherembodiments, the functionality of the electrode domain may beincorporated into the bioprotective domain so as to provide a unitarydomain that includes the functionality of the bioprotective domain,diffusion resistance domain, enzyme domain, and electrode domain.

In some embodiments, the electrode domain is located most proximal tothe electrochemically reactive surfaces. To facilitate electrochemicalreaction, the electrode domain may include a semipermeable coating thatmaintains hydrophilicity at the electrochemically reactive surfaces ofthe sensor interface. The electrode domain can enhance the stability ofan adjacent domain by protecting and supporting the material that makesup the adjacent domain. The electrode domain may also assist instabilizing the operation of the device by overcoming electrode start-upproblems and drifting problems caused by inadequate electrolyte. Thebuffered electrolyte solution contained in the electrode domain may alsoprotect against pH-mediated damage that can result from the formation ofa large pH gradient between the substantially hydrophobic interferencedomain and the electrodes due to the electrochemical activity of theelectrodes.

In some embodiments, the electrode domain includes a flexible,water-swellable, substantially solid gel-like film (e.g., a hydrogel)having a ‘dry film’ thickness of from about 0.05 microns to about 100microns, and in certain embodiments from about 0.05, 0.1, 0.15, 0.2,0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1 microns to about 1.5, 2, 2.5, 3, or3.5, 4, 4.5, 5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12,13, 14, 15, 16, 17, 18, 19, 19.5, 20, 30, 40, 50, 60, 70, 80, 90, or 100microns. In some embodiments, the thickness of the electrode domain maybe from about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns inthe case of a transcutaneously implanted sensor, or from about 6, 7, or8 microns to about 9, 10, 11, or 12 microns in the case of a whollyimplanted sensor. The term ‘dry film thickness’ as used herein is abroad term, and is to be given its ordinary and customary meaning to aperson of ordinary skill in the art (and is not to be limited to aspecial or customized meaning), and refers without limitation to thethickness of a cured film cast from a coating formulation onto thesurface of the membrane by standard coating techniques. The coatingformulation may comprise a premix of film-forming polymers and acrosslinking agent and may be curable upon the application of moderateheat.

In certain embodiments, the electrode domain may be formed of a curablemixture of a urethane polymer and a hydrophilic polymer. In some ofthese embodiments, coatings are formed of a polyurethane polymer havinganionic carboxylate functional groups and non-ionic hydrophilicpolyether segments, which are crosslinked in the presence ofpolyvinylpyrrolidone and cured at a moderate temperature of about 50° C.

Particularly suitable for this purpose are aqueous dispersions offully-reacted colloidal polyurethane polymers having cross-linkablecarboxyl functionality (e.g., BAYBOND®; Mobay Corporation). Thesepolymers are supplied in dispersion grades having apolycarbonate-polyurethane backbone containing carboxylate groupsidentified as XW-121 and XW-123; and a polyester-polyurethane backbonecontaining carboxylate groups, identified as XW-110-2. In someembodiments, BAYBOND® 123, an aqueous anionic dispersion of an aliphatepolycarbonate urethane polymer sold as a 35 weight percent solution inwater and co-solvent N-methyl-2-pyrrolidone, may be used.

In some embodiments, the electrode domain is formed from a hydrophilicpolymer that renders the electrode domain substantially more hydrophilicthan an overlying domain (e.g., interference domain, enzyme domain).Such hydrophilic polymers may include, a polyamide, a polylactone, apolyimide, a polylactam, a functionalized polyamide, a functionalizedpolylactone, a functionalized polyimide, a functionalized polylactam orcombinations thereof, for example.

In some embodiments, the electrode domain is formed primarily from ahydrophilic polymer, and in some of these embodiments, the electrodedomain is formed substantially from PVP. PVP is a hydrophilicwater-soluble polymer and is available commercially in a range ofviscosity grades and average molecular weights ranging from about 18,000to about 500,000, under the PVP homopolymer series by BASF Wyandotte andby GAF Corporation. In certain embodiments, a PVP homopolymer having anaverage molecular weight of about 360,000 identified as PVP-K90 (BASFWyandotte) may be used to form the electrode domain. Also suitable arehydrophilic, film-forming copolymers of N-vinylpyrrolidone, such as acopolymer of N-vinylpyrrolidone and vinyl acetate, a copolymer ofN-vinylpyrrolidone, ethylmethacrylate and methacrylic acid monomers, andthe like.

In certain embodiments, the electrode domain is formed entirely from ahydrophilic polymer. Useful hydrophilic polymers contemplated include,but are not limited to, poly-N-vinylpyrrolidone,poly-N-vinyl-2-piperidone, poly-N-vinyl-2-caprolactam,poly-N-vinyl-3-methyl-2-caprolactam, poly-N-vinyl-3-methyl-2-piperidone,poly-N-vinyl-4-methyl-2-piperidone, poly-N-vinyl-4-methyl-2-caprolactam,poly-N-vinyl-3-ethyl-2-pyrrolidone,poly-N-vinyl-4,5-dimethyl-2-pyrrolidone, polyvinylimidazole,poly-N,N-dimethylacrylamide, polyvinyl alcohol, polyacrylic acid,polyethylene oxide, poly-2-ethyl-oxazoline, copolymers thereof andmixtures thereof. A blend of two or more hydrophilic polymers may bepreferred in some embodiments.

It is contemplated that in certain embodiments, the hydrophilic polymerused may not be crosslinked, but in other embodiments, crosslinking maybe used and achieved by any of a variety of methods, for example, byadding a crosslinking agent. In some embodiments, a polyurethane polymermay be crosslinked in the presence of PVP by preparing a premix of thepolymers and adding a cross-linking agent just prior to the productionof the membrane. Suitable cross-linking agents contemplated include, butare not limited to, carbodiimides (e.g.,1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, UCARLNK®.XL-25 (Union Carbide)), epoxides and melamine/formaldehyde resins.Alternatively, it is also contemplated that crosslinking may be achievedby irradiation at a wavelength sufficient to promote crosslinkingbetween the hydrophilic polymer molecules, which is believed to create amore tortuous diffusion path through the domain.

The flexibility and hardness of the coating can be varied as desired byvarying the dry weight solids of the components in the coatingformulation. The term ‘dry weight solids’ as used herein is a broadterm, and is to be given its ordinary and customary meaning to a personof ordinary skill in the art (and is not to be limited to a special orcustomized meaning), and refers without limitation to the dry weightpercent based on the total coating composition after the time thecrosslinker is included. In one embodiment, a coating formulation cancontain about 6 to about 20 dry weight percent, preferably about 8 dryweight percent, PVP; about 3 to about 10 dry weight percent, in certainembodiments about 5 dry weight percent cross-linking agent; and about 70to about 91 weight percent, in certain embodiments about 87 weightpercent of a polyurethane polymer, such as a polycarbonate-polyurethanepolymer, for example. The reaction product of such a coating formulationis referred to herein as a water-swellable cross-linked matrix ofpolyurethane and PVP.

In some embodiments, underlying the electrode domain is an electrolytephase that when hydrated is a free-fluid phase including a solutioncontaining at least one compound, typically a soluble chloride salt,which conducts electric current. In one embodiment wherein the membranesystem is used with a glucose sensor such as is described herein, theelectrolyte phase flows over the electrodes and is in contact with theelectrode domain. It is contemplated that certain embodiments may useany suitable electrolyte solution, including standard, commerciallyavailable solutions. Generally, the electrolyte phase can have the sameosmotic pressure or a lower osmotic pressure than the sample beinganalyzed. In preferred embodiments, the electrolyte phase comprisesnormal saline.

Bioactive Agents

It is contemplated that any of a variety of bioactive (therapeutic)agents can be used with the analyte sensor systems described herein,such as the analyte sensor system shown in FIG. 1. In some embodiments,the bioactive agent is an anticoagulant. The term ‘anticoagulant’ asused herein is a broad term, and is to be given its ordinary andcustomary meaning to a person of ordinary skill in the art (and is notto be limited to a special or customized meaning), and refers withoutlimitation to a substance the prevents coagulation (e.g., minimizes,reduces, or stops clotting of blood). In these embodiments, theanticoagulant included in the analyte sensor system may preventcoagulation within or on the sensor. Suitable anticoagulants forincorporation into the sensor system include, but are not limited to,vitamin K antagonists (e.g., Acenocoumarol, Clorindione, Dicumarol(Dicoumarol), Diphenadione, Ethyl biscoumacetate, Phenprocoumon,Phenindione, Tioclomarol, or Warfarin), heparin group anticoagulants(e.g., Platelet aggregation inhibitors: Antithrombin III, Bemiparin,Dalteparin, Danaparoid, Enoxaparin, Heparin, Nadroparin, Parnaparin,Reviparin, Sulodexide, Tinzaparin), other platelet aggregationinhibitors (e.g., Abciximab, Acetylsalicylic acid (Aspirin), Aloxiprin,Beraprost, Ditazole, Carbasalate calcium, Cloricromen, Clopidogrel,Dipyridamole, Epoprostenol, Eptifibatide, Indobufen, Iloprost,Picotamide, Ticlopidine, Tirofiban, Treprostinil, Triflusal), enzymes(e.g., Alteplase, Ancrod, Anistreplase, Brinase, Drotrecogin alfa,Fibrinolysin, Protein C, Reteplase, Saruplase, Streptokinase,Tenecteplase, Urokinase), direct thrombin inhibitors (e.g., Argatroban,Bivalirudin, Desirudin, Lepirudin, Melagatran, Ximelagatran, otherantithrombotics (e.g., Dabigatran, Defibrotide, Dermatan sulfate,Fondaparinux, Rivaroxaban), and the like.

In one embodiment, heparin is incorporated into the analyte sensorsystem, for example by dipping or spraying. While not wishing to bebound by theory, it is believed that heparin coated on the catheter orsensor may prevent aggregation and clotting of blood on the analytesensor system, thereby preventing thromboembolization (e.g., preventionof blood flow by the thrombus or clot) or subsequent complications. Inanother embodiment, an antimicrobial is coated on the catheter (inner orouter diameter) or sensor.

In some embodiments, an antimicrobial agent may be incorporated into theanalyte sensor system. The antimicrobial agents contemplated mayinclude, but are not limited to, antibiotics, antiseptics, disinfectantsand synthetic moieties, and combinations thereof, and other agents thatare soluble in organic solvents such as alcohols, ketones, ethers,aldehydes, acetonitrile, acetic acid, methylene chloride and chloroform.The amount of each antimicrobial agent used to impregnate the medicaldevice varies to some extent, but is at least of an effectiveconcentration to inhibit the growth of bacterial and fungal organisms,such as staphylococci, gram-positive bacteria, gram-negative bacilli andCandida.

In some embodiments, an antibiotic may be incorporated into the analytesensor system. Classes of antibiotics that can be used includetetracyclines (e.g., minocycline), rifamycins (e.g., rifampin),macrolides (e.g., erythromycin), penicillins (e.g., nafeillin),cephalosporins (e.g., cefazolin), other beta-lactam antibiotics (e.g.,imipenem, aztreonam), aminoglycosides (e.g., gentamicin),chloramphenicol, sulfonamides (e.g., sulfamethoxazole), glycopeptides(e.g., vancomycin), quinolones (e.g., ciprofloxacin), fusidic acid,trimethoprim, metronidazole, clindamycin, mupirocin, polyenes (e.g.,amphotericin B), azoles (e.g., fluconazole), and beta-lactam inhibitors(e.g., sulbactam).

Examples of specific antibiotics that can be used include minocycline,rifampin, erythromycin, nafcillin, cefazolin, imipenem, aztreonam,gentamicin, sulfamethoxazole, vancomycin, ciprofloxacin, trimethoprim,metronidazole, clindamycin, teicoplanin, mupirocin, azithromycin,clarithromycin, ofloxacin, lomefloxacin, norfloxacin, nalidixic acid,sparfloxacin, pefloxacin, amifloxacin, enoxacin, fleroxacin,temafloxacin, tosufloxacin, clinafloxacin, sulbactam, clavulanic acid,amphotericin B, fluconazole, itraconazole, ketoconazole, and nystatin.

In some embodiments, an antiseptic or disinfectant may be incorporatedinto the analyte sensor system. Examples of antiseptics anddisinfectants are hexachlorophene, cationic bisiguanides (e.g.,chlorhexidine, cyclohexidine) iodine and iodophores (e.g.,povidoneiodine), para-chloro-meta-xylenol, triclosan, furan medicalpreparations (e.g., nitrofurantoin, nitrofurazone), methenamine,aldehydes (glutaraldehyde, formaldehyde) and alcohols. Other examples ofantiseptics and disinfectants will readily suggest themselves to thoseof ordinary skill in the art.

In some embodiments, an anti-barrier cell agent may be incorporated intothe analyte sensor system. Anti-barrier cell agents may includecompounds exhibiting affects on macrophages and foreign body giant cells(FBGCs). It is believed that anti-barrier cell agents prevent closure ofthe barrier to solute transport presented by macrophages and FBGCs atthe device-tissue interface during FBC maturation. Anti-barrier cellagents may provide anti-inflammatory or immunosuppressive mechanismsthat affect the wound healing process, for example, healing of the woundcreated by the incision into which an implantable device is inserted.Cyclosporine, which stimulates very high levels of neovascularizationaround biomaterials, can be incorporated into a bioprotective membraneof a preferred embodiment (see, e.g., U.S. Pat. No. 5,569,462 toMartinson et al.). Alternatively, Dexamethasone, which abates theintensity of the FBC response at the tissue-device interface, can beincorporated into a bioprotective membrane of a preferred embodiment.Alternatively, Rapamycin, which is a potent specific inhibitor of somemacrophage inflammatory functions, can be incorporated into abioprotective membrane of a preferred embodiment.

In some embodiments, an, anti-inflammatory agent may be incorporatedinto the analyte sensor system to reduce acute or chronic inflammationadjacent to the implant or to decrease the formation of a FBC capsule toreduce or prevent barrier cell layer formation, for example. Suitableanti-inflammatory agents include but are not limited to, for example,nonsteroidal anti-inflammatory drugs (NSAIDs) such as acetometaphen,aminosalicylic acid, aspirin, celecoxib, choline magnesiumtrisalicylate, diclofenac potassium, diclofenac sodium, diflunisal,etodolac, fenoprofen, flurbiprofen, ibuprofen, indomethacin, interleukin(IL)-10, IL-6 mutein, anti-IL-6 iNOS inhibitors (for example, L-NAME orL-NMDA), Interferon, ketoprofen, ketorolac, leflunomide, melenamic acid,mycophenolic acid, mizoribine, nabumetone, naproxen, naproxen sodium,oxaprozin, piroxicam, rofecoxib, salsalate, sulindac, and tolmetin; andcorticosteroids such as cortisone, hydrocortisone, methylprednisolone,prednisone, prednisolone, betamethesone, beclomethasone dipropionate,budesonide, dexamethasone sodium phosphate, flunisolide, fluticasonepropionate, paclitaxel, tacrolimus, tranilast, triamcinolone acetonide,betamethasone, fluocinolone, fluocinonide, betamethasone dipropionate,betamethasone valerate, desonide, desoximetasone, fluocinolone,triamcinolone, triamcinolone acetonide, clobetasol propionate, anddexamethasone.

In some embodiments, an immunosuppressive or immunomodulatory agent maybe incorporated into the analyte sensor system in order to interferedirectly with several key mechanisms necessary for involvement ofdifferent cellular elements in the inflammatory response. Suitableimmunosuppressive and immunomodulatory agents include, but are notlimited to, anti-proliferative, cell-cycle inhibitors, (for example,paclitaxel, cytochalasin D, infiximab), taxol, actinomycin, mitomycin,thospromote VEGF, estradiols, NO donors, QP-2, tacrolimus, tranilast,actinomycin, everolimus, methothrexate, mycophenolic acid, angiopeptin,vincristing, mitomycine, statins, C MYC antisense, sirolimus (andanalogs), RestenASE, 2-chloro-deoxyadenosine, PCNA Ribozyme, batimstat,prolyl hydroxylase inhibitors, PPARγ ligands (for example troglitazone,rosiglitazone, pioglitazone), halofuginone, C-proteinase inhibitors,probucol, BCP671, EPC antibodies, catchins, glycating agents, endothelininhibitors (for example, Ambrisentan, Tesosentan, Bosentan), Statins(for example, Cerivasttin), E. coli heat-labile enterotoxin, andadvanced coatings.

In some embodiments, an anti-infective agent may be incorporated intothe analyte sensor system. In general, anti-infective agents aresubstances capable of acting against infection by inhibiting the spreadof an infectious agent or by killing the infectious agent outright,which can serve to reduce an immuno-response without an inflammatoryresponse at the implant site, for example. Anti-infective agentsinclude, but are not limited to, anthelmintics (e.g., mebendazole),antibiotics (e.g., aminoclycosides, gentamicin, neomycin, tobramycin),antifungal antibiotics (e.g., amphotericin b, fluconazole, griseofulvin,itraconazole, ketoconazole, nystatin, micatin, tolnaftate),cephalosporins (e.g., cefaclor, cefazolin, cefotaxime, ceftazidime,ceftriaxone, cefuroxime, cephalexin), beta-lactam antibiotics (e.g.,cefotetan, meropenem), chloramphenicol, macrolides (e.g., azithromycin,clarithromycin, erythromycin), penicillins (e.g., penicillin G sodiumsalt, amoxicillin, ampicillin, dicloxacillin, nafcillin, piperacillin,ticarcillin), tetracyclines (e.g., doxycycline, minocycline,tetracycline), bacitracin, clindamycin, colistimethate sodium, polymyxinb sulfate, vancomycin, antivirals (e.g., acyclovir, amantadine,didanosine, efavirenz, foscarnet, ganciclovir, indinavir, lamivudine,nelfinavir, ritonavir, saquinavir, silver, stavudine, valacyclovir,valganciclovir, zidovudine), quinolones (e.g., ciprofloxacin,levofloxacin); sulfonamides (e.g., sulfadiazine, sulfisoxazole),sulfones (e.g., dapsone), furazolidone, metronidazole, pentamidine,sulfanilamidum crystallinum, gatifloxacin, andsulfamethoxazole/trimethoprim.

In some embodiments, a vascularization agent may be incorporated intothe analyte sensor system. Vascularization agents generally may includesubstances with direct or indirect angiogenic properties. In some cases,vascularization agents may additionally affect formation of barriercells in vivo. By indirect angiogenesis, it is meant that theangiogenesis can be mediated through inflammatory or immune stimulatorypathways. It is not fully known how agents that induce localvascularization indirectly inhibit barrier-cell formation; however,while not wishing to be bound by theory, it is believed that somebarrier-cell effects can result indirectly from the effects ofvascularization agents.

Vascularization agents may provide mechanisms that promoteneovascularization and accelerate wound healing around the membrane orminimize periods of ischemia by increasing vascularization close to thetissue-device interface. Sphingosine-1-Phosphate (S1P), a phospholipidpossessing potent angiogenic activity, may be incorporated into thebioprotective membrane. Monobutyrin, a vasodilator and angiogenic lipidproduct of adipocytes, may also be incorporated into the bioprotectivemembrane. In another embodiment, an anti-sense molecule (for example,thrombospondin-2 anti-sense), which may increase vascularization, isincorporated into a bioprotective membrane.

Vascularization agents may provide mechanisms that promote inflammation,which is believed to cause accelerated neovascularization and woundhealing in vivo. In one embodiment, a xenogenic carrier, for example,bovine collagen, which by its foreign nature invokes an immune response,stimulates neovascularization, and is incorporated into a bioprotectivemembrane of some embodiments. In another embodiment, Lipopolysaccharide,an immunostimulant, may be incorporated into a bioprotective membrane.In another embodiment, a protein, for example, a bone morphogeneticprotein (BMP), which is known to modulate bone healing in tissue, may beincorporated into the bioprotective membrane.

In some embodiments, an angiogenic agent may be incorporated into theanalyte sensor system. Angiogenic agents are substances capable ofstimulating neovascularization, which can accelerate and sustain thedevelopment of a vascularized tissue bed at the tissue-device interface,for example. Angiogenic agents include, but are not limited to, BasicFibroblast Growth Factor (bFGF), (also known as Heparin Binding GrowthFactor-II and Fibroblast Growth Factor II), Acidic Fibroblast GrowthFactor (aFGF), (also known as Heparin Binding Growth Factor-I andFibroblast Growth Factor-I), Vascular Endothelial Growth Factor (VEGF),Platelet Derived Endothelial Cell Growth Factor BB (PDEGF-BB),Angiopoietin-1, Transforming Growth Factor Beta (TGF-β), TransformingGrowth Factor Alpha (TGF-Alpha), Hepatocyte Growth Factor, TumorNecrosis Factor-Alpha (TNFα), Placental Growth Factor (PLGF),Angiogenin, Interleukin-8 (IL-8), Hypoxia Inducible Factor-I (HIF-1),Angiotensin-Converting Enzyme (ACE) Inhibitor Quinaprilat, Angiotropin,Thrombospondin, Peptide KGHK, Low Oxygen Tension, Lactic Acid, Insulin,Copper Sulphate, Estradiol, prostaglandins, cox inhibitors, endothelialcell binding agents (for example, decorin or vimentin), glenipin,hydrogen peroxide, nicotine, and Growth Hormone.

In some embodiments, a pro-inflammatory agent may be incorporated intothe analyte sensor system. Pro-inflammatory agents are generallysubstances capable of stimulating an immune response in host tissue,which can accelerate or sustain formation of a mature vascularizedtissue bed. For example, pro-inflammatory agents are generally irritantsor other substances that induce chronic inflammation and chronicgranular response at the wound-site. While not wishing to be bound bytheory, it is believed that formation of high tissue granulation inducesblood vessels, which supply an adequate or rich supply of analytes tothe device-tissue interface. Pro-inflammatory agents include, but arenot limited to, xenogenic carriers, Lipopolysaccharides, S. aureuspeptidoglycan, and proteins.

These bioactive agents can be used alone or in combination. Thebioactive agents can be dispersed throughout the material of the sensor,for example, incorporated into at least a portion of the membranesystem, or incorporated into the device (e.g., housing) and adapted todiffuse through the membrane.

There are a variety of systems and methods by which a bioactive agentmay be incorporated into the sensor membrane. In some embodiments, thebioactive agent may be incorporated at the time of manufacture of themembrane system. For example, the bioactive agent can be blended priorto curing the membrane system, or subsequent to membrane systemmanufacture, for example, by coating, imbibing, solvent-casting, orsorption of the bioactive agent into the membrane system. Although insome embodiments the bioactive agent is incorporated into the membranesystem, in other embodiments the bioactive agent can be administeredconcurrently with, prior to, or after insertion of the device in vivo,for example, by oral administration, or locally, by subcutaneousinjection near the implantation site. A combination of bioactive agentincorporated in the membrane system and bioactive agent administrationlocally or systemically can be preferred in certain embodiments.

In general, a bioactive agent can be incorporated into the membranesystem, or incorporated into the device and adapted to diffusetherefrom, in order to modify the in vivo response of the host to themembrane. In some embodiments, the bioactive agent may be incorporatedonly into a portion of the membrane system adjacent to the sensingregion of the device, over the entire surface of the device except overthe sensing region, or any combination thereof, which can be helpful incontrolling different mechanisms or stages of in vivo response (e.g.,thrombus formation). In some alternative embodiments however, thebioactive agent may be incorporated into the device proximal to themembrane system, such that the bioactive agent diffuses through themembrane system to the host circulatory system.

The bioactive agent can include a carrier matrix, wherein the matrixincludes one or more of collagen, a particulate matrix, a resorbable ornon-resorbable matrix, a controlled-release matrix, or a gel. In someembodiments, the carrier matrix includes a reservoir, wherein abioactive agent is encapsulated within a microcapsule. The carriermatrix can include a system in which a bioactive agent is physicallyentrapped within a polymer network. In some embodiments, the bioactiveagent is cross-linked with the membrane system, while in others thebioactive agent is sorbed into the membrane system, for example, byadsorption, absorption, or imbibing. The bioactive agent can bedeposited in or on the membrane system, for example, by coating,filling, or solvent casting. In certain embodiments, ionic and nonionicsurfactants, detergents, micelles, emulsifiers, demulsifiers,stabilizers, aqueous and oleaginous carriers, solvents, preservatives,antioxidants, or buffering agents are used to incorporate the bioactiveagent into the membrane system. The bioactive agent can be incorporatedinto a polymer using techniques such as described above, and the polymercan be used to form the membrane system, coatings on the membranesystem, portions of the membrane system, or any portion of the sensorsystem.

The membrane system can be manufactured using techniques known in theart. The bioactive agent can be sorbed into the membrane system, forexample, by soaking the membrane system for a length of time (forexample, from about an hour or less to about a week, or more preferablyfrom about 4, 8, 12, 16, or 20 hours to about 1, 2, 3, 4, 5, or 7 days).

The bioactive agent can be blended into uncured polymer prior to formingthe membrane system. The membrane system is then cured and the bioactiveagent thereby cross-linked or encapsulated within the polymer that formsthe membrane system.

In yet another embodiment, microspheres are used to encapsulate thebioactive agent. The microspheres can be formed of biodegradablepolymers, most preferably synthetic polymers or natural polymers such asproteins and polysaccharides. As used herein, the term polymer is usedto refer to both to synthetic polymers and proteins. U.S. Pat. No.6,281,015, discloses some systems and methods that can be used inconjunction with the preferred embodiments. In general, bioactive agentscan be incorporated in (1) the polymer matrix forming the microspheres,(2) microparticle(s) surrounded by the polymer which forms themicrospheres, (3) a polymer core within a protein microsphere, (4) apolymer coating around a polymer microsphere, (5) mixed in withmicrospheres aggregated into a larger form, or (6) a combinationthereof. Bioactive agents can be incorporated as particulates or byco-dissolving the factors with the polymer. Stabilizers can beincorporated by addition of the stabilizers to the factor solution priorto formation of the microspheres.

The bioactive agent can be incorporated into a hydrogel and coated orotherwise deposited in or on the membrane system. Some hydrogelssuitable for use in the preferred embodiments include cross-linked,hydrophilic, three-dimensional polymer networks that are highlypermeable to the bioactive agent and are triggered to release thebioactive agent based on a stimulus.

The bioactive agent can be incorporated into the membrane system bysolvent casting, wherein a solution including dissolved bioactive agentis disposed on the surface of the membrane system, after which thesolvent is removed to form a coating on the membrane surface.

The bioactive agent can be compounded into a plug of material, which isplaced within the device, such as is described in U.S. Pat. No.4,506,680 and U.S. Pat. No. 5,282,844. In some embodiments, it ispreferred to dispose the plug beneath a membrane system; in this way,the bioactive agent is controlled by diffusion through the membrane,which provides a mechanism for sustained-release of the bioactive agentin the host.

Release of Bioactive Agents

Numerous variables can affect the pharmacokinetics of bioactive agentrelease. The bioactive agents of the preferred embodiments can beoptimized for short- or long-term release. In some embodiments, thebioactive agents of the preferred embodiments are designed to aid orovercome factors associated with short-term effects (e.g., acuteinflammation or thrombosis) of sensor insertion. In some embodiments,the bioactive agents of the preferred embodiments are designed to aid orovercome factors associated with long-term effects, for example, chronicinflammation or build-up of fibrotic tissue or plaque material. In someembodiments, the bioactive agents of the preferred embodiments combineshort- and long-term release to exploit the benefits of both.

As used herein, ‘controlled,’ sustained or ‘extended’ release of thefactors can be continuous or discontinuous, linear or non-linear. Thiscan be accomplished using one or more types of polymer compositions,drug loadings, selections of excipients or degradation enhancers, orother modifications, administered alone, in combination or sequentiallyto produce the desired effect.

Short-term release of the bioactive agent in the preferred embodimentsgenerally refers to release over a period of from about a few minutes orhours to about 2, 3, 4, 5, 6, or 7 days or more.

Loading of Bioactive Agents

The amount of loading of the bioactive agent into the membrane systemcan depend upon several factors. For example, the bioactive agent dosageand duration can vary with the intended use of the membrane system, forexample, the intended length of use of the device and the like;differences among patients in the effective dose of bioactive agent;location and methods of loading the bioactive agent; and release ratesassociated with bioactive agents and optionally their carrier matrix.Therefore, one skilled in the art will appreciate the variability in thelevels of loading the bioactive agent, for the reasons described above.

In some embodiments, in which the bioactive agent is incorporated intothe membrane system without a carrier matrix, the preferred level ofloading of the bioactive agent into the membrane system can varydepending upon the nature of the bioactive agent. The level of loadingof the bioactive agent is preferably sufficiently high such that abiological effect (e.g., thrombosis prevention) is observed. Above thisthreshold, the bioactive agent can be loaded into the membrane system soas to imbibe up to 100% of the solid portions, cover all accessiblesurfaces of the membrane, or fill up to 100% of the accessible cavityspace. Typically, the level of loading (based on the weight of bioactiveagent(s), membrane system, and other substances present) is from about 1ppm or less to about 1000 ppm or more, preferably from about 2, 3, 4, or5 ppm up to about 10, 25, 50, 75, 100, 200, 300, 400, 500, 600, 700,800, or 900 ppm. In certain embodiments, the level of loading can be 1wt. % or less up to about 50 wt. % or more, preferably from about 2, 3,4, 5, 6, 7, 8, 9, 10, 15, or 20 wt. % up to about 25, 30, 35, 40, or 45wt. %.

When the bioactive agent is incorporated into the membrane system with acarrier matrix, such as a gel, the gel concentration can be optimized,for example, loaded with one or more test loadings of the bioactiveagent. It is generally preferred that the gel contain from about 0.1 orless to about 50 wt. % or more of the bioactive agent(s), preferablyfrom about 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 wt. % to about 6,7, 8, 9, 10, 15, 20, 25, 30, 35, 40, or 45 wt. % or more bioactiveagent(s), more preferably from about 1, 2, or 3 wt. % to about 4 or 5wt. % of the bioactive agent(s). Substances that are not bioactive canalso be incorporated into the matrix.

Referring now to microencapsulated bioactive agents, the release of theagents from these polymeric systems generally occurs by two differentmechanisms. The bioactive agent can be released by diffusion throughaqueous filled channels generated in the dosage form by the dissolutionof the agent or by voids created by the removal of the polymer solventor a pore forming agent during the original micro-encapsulation.Alternatively, release can be enhanced due to the degradation of theencapsulating polymer. With time, the polymer erodes and generatesincreased porosity and microstructure within the device. This createsadditional pathways for release of the bioactive agent.

In some embodiments, the sensor is designed to be bioinert, e.g., by theuse of bioinert materials. Bioinert materials do not substantially causeany response from the host. As a result, cells can live adjacent to thematerial but do not form a bond with it. Bioinert materials include butare not limited to alumina, zirconia, titanium oxide or other bioinertmaterials generally used in the ‘catheter/catheterization’ art. Whilenot wishing to be bound by theory, it is believed that inclusion of abioinert material in or on the sensor can reduce attachment of bloodcells or proteins to the sensor, thrombosis or other host reactions tothe sensor.

EXAMPLES Example 1

Sensors were built to test the ability of a silicone endgroup-containing polyurethane to reduce or block non-constant noise on aglucose sensor signal. Transcutaneous sensors, with electrode, enzymeand bioprotective domains, were built and tested. The control and testsensors were built as described in the section entitled ‘ExemplaryGlucose Sensor Configuration,’ including an electrode domain, an enzymedomain and an integral bioprotective domain with one difference: thetest sensors were built with a bioprotective domain comprising asilicone-polycarbonate-urethane including about 19% silicone by weight,and further including PVP added thereto (about 25% by weight to provideglucose permeability to the membrane); and the control sensors werebuilt with a bioprotective domain comprising a polyurethane membranewith both hydrophilic and hydrophobic regions to control the diffusionof glucose and oxygen to the glucose sensor. Namely, the bioprotectivedomain of the test sensors included a polyurethane with silicone endgroups (˜19% by weight silicone) as compared to the control sensors,which did not include silicone in the bioprotective domain.

Six of the control sensors and six of the test sensors were placed in asolution containing 200 mg/dL glucose, and then subsequently placed in asolution containing 200 mg/dL of glucose and a therapeutic does ofacetaminophen (165 μM). When the control sensors were moved to theglucose and acetaminophen containing solution, the signal increased onaverage by 622%. When the test sensors were moved to the glucose andacetaminophen containing solution, the signal increased on average by4%. Accordingly, a glucose sensor having a bioprotective domaincomprising a silicone end group-containing polyurethane, including about19% silicone by weight, blended with PVP may substantially block orattenuate the effect or influence of a known interferent, acetaminophen,as compared to a control sensor.

Example 2

Test and Control sensors as described with reference to Example 1,above, were implanted bilaterally in humans and the signal evaluated.FIG. 5 is a graph illustrating the continuous glucose sensor data fromthe bilaterally implanted sensors in one human host over about two days.The x-axis represents time; the y-axis represents signal amplitude incounts. Circles represent the data set obtained from a control sensorwith the configuration of Example 1 implanted on a first side of thehost. The squares represent the data set obtained from a test sensorwith the configuration of Example 1 implanted on the other side of thesame host. It can be seen that the control sensor exemplified a muchhigher level of (non-constant) noise than the test sensor, as evidencedby the sporadic rises and falls seen in the control sensor data duringthe first 24 hours, for example. These rises and falls arenon-physiological in nature, as evidenced by their rate of change beingabove known physiological limits of glucose concentration in humans.After about 24 hours, the host ingested a therapeutic dose ofacetaminophen. The spike (indicated by the arrow) in the control sensordata correlates with the acetaminophen ingestion while thetime-corresponding test sensor data (associated with the timing of theacetaminophen ingestion) does not show a substantial change in thesignal. Accordingly, a bioprotective domain comprising a silicone endgroup-containing polyurethane, including about 19% silicone by weight,substantially blocks or attenuates the affect and/or influence of aknown chemical and biological non-constant noise-causing species.

Example 3

Test and Control sensors as described with reference to Example 1,above, were implanted bilaterally in diabetic rats for more than about 2days. FIGS. 6A and 6B illustrate exemplary test results from a controlsensor (FIG. 6A) and test sensor (FIG. 6B) implanted bilaterally in onerat, over a period more than about 2 days, after sensor break-in. TheY-axis represents signal amplitude (in counts). The X-axis representstime. Double-headed arrows approximately indicate the days of the study.The total signal detected by the test glucose sensor is shown as filleddiamonds. To determine the signal components, the total signal, for eachof the test and control data sets, was analyzed in the following manner.First, the total signal was filtered using an IIR filter to obtain thefiltered signal (open diamonds). The non-constant noise component(filled circles) was obtained by subtracting the filtered signal fromthe total signal. Next, the filtered signal was calibrated using glucosevalues obtained from a finger-stick glucose meter (SHBG), as describedas described in more detail elsewhere herein, to obtain the constantnoise signal component (e.g., from the baseline of the calibrationequation, not shown). Finally, the glucose component (open circles) ofthe total signal was obtained by subtracting the constant noise signalcomponent from the filtered signal.

A severe noise episode can be seen on Day 1 (from about 15:30 to about18:50) on the control sensor data set (FIG. 6A). During the noiseepisode the non-constant noise component of the signal from the controlsensor was about 21.8% of the total signal as compared to thenon-constant noise component of the signal from the test sensor was onlyabout 2.4% of the total signal. Using the Root Mean Square (RMS) methodwith a window of about 3 hours and 15 minutes, it was determined thatthe non-constant noise signal component was no more than about 12% ofthe total signal for the test sensor (including the bioprotective domainof the preferred embodiments) at any time during the sensor session.Accordingly, it was shown that a sensor including a bioprotective domainof the preferred embodiments (including a silicone end group-containingpolyurethane) can reduce the non-constant noise-component of the totalsignal by about 18% during a severe noise episode. Furthermore, it wasshown that for a glucose sensor including a bioprotective domain of thepreferred embodiments, the non-constant noise component of the signal isless than about 12% of the total signal over a period of more than abouta 2-day sensor session.

Example 4

An analysis was conducted on test sensors, which were built insubstantially the same way as the test sensors described in Example 1,to determine whether a strong positive correlation exists between invivo and in vitro sensor glucose measurements (e.g., sensitivity ofglucose concentration readings). The test sensors were built withelectrode, enzyme, and bioprotective domains. The bioprotective domainincluded a silicone-polycarbonate-urethane having about 20% silicone byweight, and further included PVP added thereto (about 17.5% by weight toprovide glucose permeability to the membrane). A number of the testsensors were placed in glucose PBS (phosphate buffered saline) solutionfor calibration use, while a corresponding number of test sensors werethen implanted in vivo into diabetic rats for more than about seven daysto monitor their glucose levels. FIG. 7 illustrates a graph comparingthe initial in vivo glucose sensitivity of a test sensor implanted inone rat with the in vitro glucose sensitivity of a test sensor inglucose PBS solution. As shown in FIG. 7, a linear regression was thenperformed to calculate the sensitivities of the test sensors in an invivo environment and in an in vitro environment. The sensitivities ofthe in vivo and the in vitro test sensors were found to be about 13.37and 13.73 pA/mg/dL, respectively. Accordingly, it can be determined thatthe ratio between in vivo and in vitro glucose sensitivities in thisparticular study was at least greater than 0.97 to 1, and about 1 to 1,with a standard deviation of about 0.1. The test data also showed thatthe correlation, i.e., R², between in vivo and in vitro glucosesensitivities of a fixed population of test sensors manufactured insubstantially the same way to be about 0.98.

In similar studies, while the in vivo to in vitro sensitivity ratio wasnot found to be 1 to 1, the in vivo to in vitro sensitivity ratio wasnonetheless found to be substantially fixed. In other words, in thesestudies, the ratio was found to be substantially consistent across afixed population of test sensors manufactured in substantially the sameway. In these studies, the ratios between in vivo and in vitro glucosesensitivities have been found in certain circumstances to be from about1 to 1.5 to about 1 to 10, in other circumstances from about 1 to 0.1and about 1 to 0.7. In these studies, the correlation between in vivoand in vitro glucose sensitivities was also found to be high, i.e., incertain circumstances greater than or about 0.7, in other circumstancesgreater than or about 0.8, in still other circumstances greater than orabout 0.9, in certain circumstances, greater than or about 0.95, and instill other circumstances greater than or about 0.98.

Example 5

Dual-electrode sensors were built to test the ability of a silicone endgroup-containing polyurethane blended with PVP to reduce or blocknon-constant noise on a glucose sensor signal. The dual-electrodesensors were each built to include an electrode layer, an enzyme layerand a bioprotective layer. (As described below, in some instances, someor all of the enzyme layer did not include enzyme). More specifically,the dual-electrode sensors were constructed from two platinum wires,each coated with a layer of polyurethane to form the electrode layer.Exposed electroactive windows were cut into the wires by removing aportion thereof. The sensors were trimmed to a length. A solution withthe glucose oxidase enzyme was then applied to one electrode (i.e., theenzymatic electrode) to form an enzyme layer, while the same solution,but without glucose oxidase, was then applied to the other electrode(i.e., the non-enzymatic electrode) to form a non-enzyme layer. Afterthe sensors were dried, a bioprotective layer was deposited onto eachsensor and then dried. Depending on whether a particular sensor wasassigned as a control sensor or as a test sensor, the material depositedonto the sensor to form the bioprotective layer was different. Withcontrol sensors, the bioprotective layer was formed of a conventionalpolyurethane membrane. In contrast, with test sensors, the bioprotectivelayer was formed of a blend of silicone-polycarbonate-urethane(approximately 84% by weight) and polyvinylpyrrolidone (16% by weight).The platinum wires were then laid next to each other such that thewindows are offset (e.g., separated by a diffusion barrier). The bundlewas then placed into a winding machine and silver wire was wrappedaround the platinum electrodes. The silver wire was then chloridized toproduce a silver/silver chloride reference electrode.

FIG. 8 illustrates the results from one in vivo experiment comparing thesignals received from the enzymatic electrodes of the test and controlsensors. During testing, the test and control sensors were incorporatedinto catheters connected to human patients and to an intravenous bloodglucose monitoring system, and a 1,000 mg dose of acetaminophen wasadministered orally to the patients. As illustrated in FIG. 8, thepatients linked to the control and test sensors were each administeredwith the acetaminophen dose at approximately 11:48 AM. As alsoillustrated, after the patient linked to the test sensor wasadministered acetaminophen, the signals received from the enzymaticelectrode ascended from readings of about 105-115 mg/dL to readings ofabout 185-195 mg/dL. From this, it can be estimated that for the controlsensor in this particular experiment, the equivalent peak glucoseresponse of the enzymatic electrode to a 1,000 mg dose of acetaminophenadministered to the patient is at least about 80 mg/dL. To compare, asalso illustrated in FIG. 8, after the other patient linked with thecontrol sensor was administered acetaminophen, the baseline signalsreceived from the enzymatic electrode quickly increased from readings ofabout 70-80 mg/dL to readings of about 390-400 mg/dL. From this, it canbe estimated that for test sensor in this particular experiment, theequivalent peak glucose response of the enzymatic electrode to a 1,000mg dose of acetaminophen administered to the patient is at least about320 mg/dL. Collectively, these results appear to indicate that the useof a polymer comprising a blend of a silicone-polycarbonate-urethanebase polymer with polyvinylpyrrolidone can provide a mechanism forreducing the flux of interferents (e.g., acetaminophen) through themembrane.

Example 6

An in vivo analysis was conducted to compare theglucose-signal-to-baseline-signal ratios of the control and test sensorsdescribed in Example 5. As previously described, the dual-electrodesensors in this experiment each comprise one electrode configured to beenzymatic and a corresponding electrode configured to be non-enzymatic.The enzymatic electrode is configured to measure a total signalcomprising glucose and baseline signals, and the non-enzymatic electrodeis configured to measure a baseline signal consisting of the baselinesignal only. In this way, the baseline signal can be determined andsubtracted from the total signal to generate a difference signal, i.e.,a glucose-only signal that is substantially not subject to fluctuationsin the baseline or interfering species on the signal.

To provide a basis for comparing the two sensors, data were taken at thesame glucose concentration for both sensors. In this particularexperiment, sensor data in the normal glucose range, i.e., approximately80-125 mg/dL were selected. In a first experiment, for both the controland test sensors, the glucose-signal-to-baseline-signal ratios werecalculated and compared in an environment where the glucoseconcentration is approximately 80 mg/dL and where acetaminophen was notdetectably present, as illustrated in FIG. 9A. In a second experiment,for both the control and test sensors, theglucose-signal-to-baseline-signal ratios were calculated and compared inan environment where the glucose concentration is approximately 125mg/dL and where acetaminophen was present at a concentration ofapproximately 1-3 mg/dL, as illustrated in FIG. 9B. As shown in FIGS. 9Aand 9B, under both above-described environments, the test sensor hadconsiderably higher glucose-signal-to-baseline-signal ratios than thecontrol sensor. For instance, as shown in FIG. 9A, under an environmentwhere glucose concentration was approximately 80 mg/dL and where therewas no acetaminophen detectably present, the baseline signal of the testsensor was found to be approximately 15% of the total signal(corresponding to a glucose-signal-to-baseline-signal ratio ofapproximately 5.7 to 1), whereas the baseline signal of the controlsensor was found to be approximately 53% of the total signal(corresponding to a glucose-signal-to-baseline-signal ratio ofapproximately 0.9 to 1). As also shown in FIG. 9B, under an environmentwhere glucose concentration was approximately 125 mg/dL and whereacetaminophen was present at a concentration of approximately 1-3 mg/dL,the baseline signal of the test sensor was found to be approximately 15%of the total signal (corresponding to aglucose-signal-to-baseline-signal ratio of approximately 5.7 to 1),whereas the baseline signal of the control sensor was found to beapproximately 61% of the total signal (corresponding to aglucose-signal-to-baseline-signal ratio of approximately 0.64 to 1). Inother similar experiments, a glucose-signal-to-baseline-signal ratio ofapproximately 2 to 1, 3 to 1, 4 to 1, 5 to 1, 6 to 1, 7 to 1, 8 to 1, 9to 1, and 10 to 1 have been obtained.

Example 7

In vitro tests were also conducted to evaluate the ability of the testsensors described in Examples 5 and 6 to reduce the interference effectsof various interferents, specifically, acetaminophen, albuterol,ascorbic acid, atenolol, haloperidol, lidocaine, mataproterenol,metoprolol, phenylephrine, propofol, and uric acid. During testing, eachtest sensor underwent a calibration check, after which, it was immersedin a solution comprising a test concentration of the interferent. Theresulting signal from the enzymatic electrode of each test sensor wasthen monitored. Based on known sensitivities of each test sensor, anestimated equivalent glucose signal was then calculated. The estimatedequivalent glucose signals from the tests performed on the differentinterferents are summarized in Table 1 below.

TABLE 1 Test Equivalent Concentration Glucose Signal Interferent (mg/dL)(mg/dL) Acetaminophen ~3 ~30 Albuterol ~0.04 ~−3 Ascorbic Acid ~6 ~17Atenolol ~1 ~1 Haloperidol ~0.1 ~−5 Lidocaine ~1.2 ~−3 Metaproterenol~0.001 ~1 Metoprolol ~0.5 ~−1 Phenylephrine ~4 ~10 Propofol ~0.65 ~0Uric Acid ~6 ~25

Example 8

Five transcutaneous glucose sensors were built to evaluate the level ofaccuracy (with respect to glucose concentration measurements) that canbe attained from using a silicone-containing polyurethane blended withPVP, with the bioprotective domain of each sensor having a differentpercentage of PVP. The five sensors were then implanted into fivedifferent human hosts.

Table 2 below summarizes the levels of accuracy, in terms of meanabsolute relative difference (MARD), attained by four of the fiveabove-described sensors. MARD was calculated by measuring the averagerelative difference between each of the above-described sensors andtheir corresponding reference measurements, on a percentage basis. Thereference measurements associated with calculation of the mean absoluterelative difference were determined by analysis of blood.

TABLE 2 Overall Low Glucose MARD Low Glucose MAD (40 mg/dL-400 MARD (40mg/dL-80 Sensor mg/dL, (40 mg/dL-80 mg/dL) (in units No. Days 1-7)mg/dL) of mg/dL) Sensor 1 6.8% 13.7%  7 Sensor 2 16.1% 4.9% 3 Sensor 39.6% 9.6% 6.5 Sensor 4 9.6%   6% 4.3

As illustrated above, with Sensor 1, the MARD calculated formeasurements of glucose concentrations from about 40 mg/dL to about 400mg/dL was measured to be about 6.8% and that from about 40 mg/dL toabout 80 mg/dL was measured to be about 13.7%. The mean absolutedifference (MAD) for Sensor 1 was about 7 mg/dL. With Sensor 2, the MARDcalculated for measurements of glucose concentrations from about 40mg/dL to about 400 mg/dL was about 16.1% and that from about 40 mg/dL toabout 80 mg/dL was measured to be about 4.9%. The MAD for Sensor 2 wasabout 3 mg/dL. With Sensor 3, the MARD calculated for measurements ofglucose concentrations from about 40 mg/dL to about 80 mg/dL and thatfrom about 40 mg/dL to about 80 mg/dL were about the same, i.e., atabout 9.6%. The MAD for Sensor 3 was about 4.3 mg/dL. With Sensor 4, theMARD (6%) calculated for measurements of glucose concentrations fromabout 40 mg/dL to about 80 mg/dL and was lower than that (9.6%) fromabout 40 mg/dL to about 400 mg/dL. The MAD for Sensor 4 was about 4.3mg/dL. As can be seen from the table, by employing the above-describedmethod for fabricating a bioprotective domain, it was possible toachieve a sensor capable of providing a low glucose concentrationaccuracy associated with MARD that was substantially the same. A fifthsensor (not listed in Table 2), which was configured to have a sensorsession of at least about 10 days and configured to enhance accuracy atthe end of the sensor session was found to have a 10th day-accuracylevel corresponding to a MARD of about 6.5%.

Methods and devices that are suitable for use in conjunction withaspects of the preferred embodiments are disclosed in U.S. Pat. No.4,757,022; U.S. Pat. No. 6,702,857; U.S. Pat. No. 7,632,228; U.S. Pat.No. 7,471,972; U.S. Pat. No. 6,001,067; U.S. Pat. No. 7,226,978; U.S.Pat. No. 7,134,999; U.S. Pat. No. 7,192,450; U.S. Pat. No. 7,599,726;U.S. Pat. No. 7,583,990; U.S. Pat. No. 7,379,765; U.S. Pat. No.7,108,778; U.S. Pat. No. 7,074,307; U.S. Pat. No. 6,931,327; U.S. Pat.No. 7,276,029; U.S. Pat. No. 7,081,195; U.S. Pat. No. 7,519,408; U.S.Pat. No. 7,364,592; U.S. Pat. No. 7,591,801; U.S. Pat. No. 7,460,898;U.S. Pat. No. 7,467,003; U.S. Pat. No. 7,366,556; U.S. Pat. No.7,424,318; U.S. Pat. No. 7,637,868; U.S. Pat. No. 7,657,297; U.S. Pat.No. 7,497,827; U.S. Pat. No. 7,310,544; U.S. Pat. No. 7,654,956; U.S.Pat. No. 7,651,596; U.S. Pat. No. 7,494,465; U.S. Pat. No. 7,640,048;U.S. Pat. No. 7,613,491; U.S. Pat. No. 7,615,007; U.S. Pat. No.6,741,877; U.S. Pat. No. 7,110,803; U.S. Pat. No. 6,558,321; U.S. Pat.No. 6,862,465; U.S. Pat. No. 7,136,689; and U.S. Pat. No. 4,994,167.

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All references cited herein, including but not limited to published andunpublished applications, patents, and literature references, areincorporated herein by reference in their entirety and are hereby made apart of this specification. To the extent publications and patents orpatent applications incorporated by reference contradict the disclosurecontained in the specification, the specification is intended tosupersede and/or take precedence over any such contradictory material.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing, the term ‘including’ shouldbe read to mean ‘including, without limitation’ or the like; the term‘comprising’ as used herein is synonymous with ‘including,’‘containing,’ or ‘characterized by,’ and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps; theterm ‘example’ is used to provide exemplary instances of the item indiscussion, not an exhaustive or limiting list thereof; and adjectivessuch as ‘known’, ‘normal’, ‘standard’, and terms of similar meaningshould not be construed as limiting the item described to a given timeperiod or to an item available as of a given time, but instead should beread to encompass known, normal, or standard technologies that may beavailable or known now or at any time in the future. Likewise, a groupof items linked with the conjunction ‘and’ should not be read asrequiring that each and every one of those items be present in thegrouping, but rather should be read as ‘and/or’ unless expressly statedotherwise. Similarly, a group of items linked with the conjunction ‘or’should not be read as requiring mutual exclusivity among that group, butrather should be read as ‘and/or’ unless expressly stated otherwise. Inaddition, as used in this application, the articles ‘a’ and ‘an’ shouldbe construed as referring to one or more than one (i.e., to at leastone) of the grammatical objects of the article. By way of example, ‘anelement’ means one element or more than one element.

The presence in some instances of broadening words and phrases such as‘one or more’, ‘at least’, ‘but not limited to’, or other like phrasesshall not be read to mean that the narrower case is intended or requiredin instances where such broadening phrases may be absent.

All numbers expressing quantities of ingredients, reaction conditions,and so forth used in the specification are to be understood as beingmodified in all instances by the term ‘about.’ Accordingly, unlessindicated to the contrary, the numerical parameters set forth herein areapproximations that may vary depending upon the desired propertiessought to be obtained. At the very least, and not as an attempt to limitthe application of the doctrine of equivalents to the scope of anyclaims in any application claiming priority to the present application,each numerical parameter should be construed in light of the number ofsignificant digits and ordinary rounding approaches.

Furthermore, although the foregoing has been described in some detail byway of illustrations and examples for purposes of clarity andunderstanding, it is apparent to those skilled in the art that certainchanges and modifications may be practiced. Therefore, the descriptionand examples should not be construed as limiting the scope of theinvention to the specific embodiments and examples described herein, butrather to also cover all modification and alternatives coming with thetrue scope and spirit of the invention.

What is claimed is:
 1. A device for continuous in vivo measurement of a glucose concentration, the device comprising: an implantable sensor configured to continuously measure a signal indicative of a glucose concentration in a host; an enzyme-containing membrane located over the sensor, wherein the enzyme-containing membrane comprises both hydrophilic and hydrophobic regions, and wherein the enzyme-containing membrane comprises: an enzyme configured to catalyze a reaction that has glucose as a reactant; and a polyurethane comprising a plurality of repeating hard segments and repeating soft segments, wherein a soft segment of the plurality of repeating soft segments has a molecular weight from about 200 Daltons to about 50,000 Daltons, wherein a hard segment of the plurality of repeating hard segments has a molecular weight from about 160 Daltons to about 10,000 Daltons; and sensor electronics operably connected to the sensor, wherein the sensor electronics are configured to measure a current flow produced by the sensor to generate sensor data indicative of glucose concentration.
 2. The device of claim 1, wherein the polyurethane is a polyurethaneurea.
 3. The device of claim 1, wherein the polyurethane comprises silicone.
 4. The device of claim 3, wherein the polyurethane comprises from about 5 to about 50% silicone by weight.
 5. The device of claim 1, wherein the hydrophilic region is formed of a material selected from the group consisting of polyvinyl acetate, poly(ethylene glycol), polyacrylamide, polyethylene oxide, poly ethyl acrylate, and polyvinylpyrrolidone.
 6. The device of claim 1, wherein the membrane comprises a domain configured to reduce permeation therethrough of an interfering species.
 7. The device of claim 1, wherein the domain is located between the sensor and the first domain.
 8. The device of claim 1, wherein the domain comprises a polymer comprising ionic components.
 9. The device of claim 1, wherein the domain is configured to limit diffusion of interfering species with a molecular weight greater than a molecular weight of hydrogen peroxide. 